diff --git a/src/common/mod.rs b/src/common/mod.rs
index 3a1e9c0f..915051a5 100644
--- a/src/common/mod.rs
+++ b/src/common/mod.rs
@@ -56,6 +56,7 @@ pub use self::referer::Referer;
 pub use self::referrer_policy::ReferrerPolicy;
 pub use self::retry_after::RetryAfter;
 pub use self::sec_websocket_accept::SecWebsocketAccept;
+pub use self::sec_websocket_extensions::{SecWebsocketExtensions, WebsocketExtension};
 pub use self::sec_websocket_key::SecWebsocketKey;
 pub use self::sec_websocket_version::SecWebsocketVersion;
 pub use self::server::Server;
@@ -175,6 +176,7 @@ mod referer;
 mod referrer_policy;
 mod retry_after;
 mod sec_websocket_accept;
+mod sec_websocket_extensions;
 mod sec_websocket_key;
 mod sec_websocket_version;
 mod server;
diff --git a/src/common/sec_websocket_extensions.rs b/src/common/sec_websocket_extensions.rs
new file mode 100644
index 00000000..28042c3f
--- /dev/null
+++ b/src/common/sec_websocket_extensions.rs
@@ -0,0 +1,445 @@
+use std::convert::TryFrom;
+
+use bytes::BytesMut;
+use http::header::SEC_WEBSOCKET_EXTENSIONS;
+
+use util::{Comma, FlatCsv, HeaderValueString, SemiColon};
+use {Error, Header, HeaderValue};
+
+/// `Sec-WebSocket-Extensions` header, defined in [RFC6455][RFC6455_11.3.2]
+///
+/// The `Sec-WebSocket-Extensions` header field is used in the WebSocket
+/// opening handshake.  It is initially sent from the client to the
+/// server, and then subsequently sent from the server to the client, to
+/// agree on a set of protocol-level extensions to use for the duration
+/// of the connection.
+///
+/// ## ABNF
+///
+/// ```text
+/// Sec-WebSocket-Extensions = extension-list
+/// extension-list = 1#extension
+/// extension = extension-token *( ";" extension-param )
+/// extension-token = registered-token
+/// registered-token = token
+/// extension-param = token [ "=" (token | quoted-string) ]
+///     ;When using the quoted-string syntax variant, the value
+///     ;after quoted-string unescaping MUST conform to the
+///     ;'token' ABNF.
+/// ```
+///
+/// ## Example Values
+///
+/// * `permessage-deflate` (defined in [RFC7692][RFC7692_7])
+/// * `permessage-deflate; server_max_window_bits=10`
+/// * `permessage-deflate; server_max_window_bits=10, permessage-deflate`
+///
+/// ## Example
+///
+/// ```rust
+/// # extern crate headers;
+/// use headers::SecWebsocketExtensions;
+///
+/// let extensions = SecWebsocketExtensions::from_static("permessage-deflate");
+/// ```
+///
+/// ## Splitting and Combining
+///
+/// Note that `Sec-WebSocket-Extensions` may be split or combined across multiple headers.
+/// The following are equivalent:
+/// ```text
+/// Sec-WebSocket-Extensions: foo
+/// Sec-WebSocket-Extensions: bar; baz=2
+/// ```
+/// ```text
+/// Sec-WebSocket-Extensions: foo, bar; baz=2
+/// ```
+///
+/// `SecWebsocketExtensions` splits extensions when decoding and combines them into a single
+/// value when encoding.
+///
+/// [RFC6455_11.3.2]: https://tools.ietf.org/html/rfc6455#section-11.3.2
+/// [RFC7692_7]: https://tools.ietf.org/html/rfc7692#section-7
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct SecWebsocketExtensions(pub Vec<WebsocketExtension>);
+
+impl Header for SecWebsocketExtensions {
+    fn name() -> &'static ::HeaderName {
+        &SEC_WEBSOCKET_EXTENSIONS
+    }
+
+    fn decode<'i, I: Iterator<Item = &'i HeaderValue>>(values: &mut I) -> Result<Self, Error> {
+        let extensions = values
+            .cloned()
+            .flat_map(|v| {
+                FlatCsv::<Comma>::from(v)
+                    .iter()
+                    .map(WebsocketExtension::try_from)
+                    .collect::<Vec<_>>()
+            })
+            .collect::<Result<Vec<_>, _>>()?;
+        if extensions.is_empty() {
+            Err(Error::invalid())
+        } else {
+            Ok(SecWebsocketExtensions(extensions))
+        }
+    }
+
+    fn encode<E: Extend<HeaderValue>>(&self, values: &mut E) {
+        if !self.is_empty() {
+            values.extend(std::iter::once(self.to_value()));
+        }
+    }
+}
+
+impl SecWebsocketExtensions {
+    /// Construct a `SecWebSocketExtensions` from a static string.
+    ///
+    /// ## Panic
+    ///
+    /// Panics if the static string is not a valid extensions valie.
+    pub fn from_static(s: &'static str) -> Self {
+        let value = HeaderValue::from_static(s);
+        SecWebsocketExtensions::try_from(&value).expect("valid static string")
+    }
+
+    /// Convert this `SecWebsocketExtensions` to a single `HeaderValue`.
+    pub fn to_value(&self) -> HeaderValue {
+        let values = self.0.iter().map(HeaderValue::from).collect::<FlatCsv>();
+        HeaderValue::from(&values)
+    }
+
+    /// An iterator over the `WebsocketExtension`s in `SecWebsocketExtensions` header(s).
+    pub fn iter(&self) -> impl Iterator<Item = &WebsocketExtension> {
+        self.0.iter()
+    }
+
+    /// Get the number of extensions.
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    /// Returns `true` if headers contain no extensions.
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+}
+
+impl TryFrom<&str> for SecWebsocketExtensions {
+    type Error = Error;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        let value = HeaderValue::from_str(value).map_err(|_| Error::invalid())?;
+        SecWebsocketExtensions::try_from(&value)
+    }
+}
+
+impl TryFrom<&HeaderValue> for SecWebsocketExtensions {
+    type Error = Error;
+
+    fn try_from(value: &HeaderValue) -> Result<Self, Self::Error> {
+        let mut values = std::iter::once(value);
+        SecWebsocketExtensions::decode(&mut values)
+    }
+}
+
+/// A WebSocket extension containing the name and parameters.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct WebsocketExtension {
+    name: HeaderValueString,
+    params: Vec<(HeaderValueString, Option<HeaderValueString>)>,
+}
+
+impl WebsocketExtension {
+    /// Construct a `WebSocketExtension` from a static string.
+    ///
+    /// ## Panics
+    ///
+    /// This function panics if the argument is invalid.
+    pub fn from_static(src: &'static str) -> Self {
+        WebsocketExtension::try_from(HeaderValue::from_static(src)).expect("valid static value")
+    }
+
+    /// Get the name of the extension.
+    pub fn name(&self) -> &str {
+        self.name.as_str()
+    }
+
+    /// An iterator over the parameters of this extension.
+    pub fn params(&self) -> impl Iterator<Item = (&str, Option<&str>)> {
+        self.params
+            .iter()
+            .map(|(k, v)| (k.as_str(), v.as_ref().map(|v| v.as_str())))
+    }
+}
+
+impl TryFrom<&str> for WebsocketExtension {
+    type Error = Error;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        if value.is_empty() {
+            Err(Error::invalid())
+        } else {
+            let value = HeaderValue::from_str(value).map_err(|_| Error::invalid())?;
+            WebsocketExtension::try_from(value)
+        }
+    }
+}
+
+impl TryFrom<HeaderValue> for WebsocketExtension {
+    type Error = Error;
+
+    fn try_from(value: HeaderValue) -> Result<Self, Self::Error> {
+        let csv = FlatCsv::<Comma>::from(value);
+        // More than one extension was found
+        if csv.iter().count() > 1 {
+            return Err(Error::invalid());
+        }
+
+        let params = FlatCsv::<SemiColon>::from(csv.value);
+        let mut params_iter = params.iter();
+        let name = params_iter
+            .next()
+            .ok_or_else(Error::invalid)
+            .map(parse_token)?
+            .and_then(HeaderValueString::from_str)?;
+        let params = params_iter
+            .map(|p| {
+                let mut kv = p.splitn(2, '=');
+                let key = kv
+                    .next()
+                    .ok_or_else(Error::invalid)
+                    .map(str::trim)
+                    .and_then(|k| parse_token(k).and_then(HeaderValueString::from_str))?;
+                let val = kv
+                    .next()
+                    .map(str::trim)
+                    .map(|v| parse_value(v).and_then(HeaderValueString::from_str))
+                    .transpose()?;
+                Ok((key, val))
+            })
+            .collect::<Result<Vec<_>, _>>()?;
+        Ok(WebsocketExtension { name, params })
+    }
+}
+
+impl From<&WebsocketExtension> for HeaderValue {
+    fn from(extension: &WebsocketExtension) -> Self {
+        let mut buf = BytesMut::from(extension.name.as_str().as_bytes());
+        for (key, val) in &extension.params {
+            buf.extend_from_slice(b"; ");
+            buf.extend_from_slice(key.as_str().as_bytes());
+            if let Some(val) = val {
+                buf.extend_from_slice(b"=");
+                buf.extend_from_slice(val.as_str().as_bytes());
+            }
+        }
+
+        HeaderValue::from_maybe_shared(buf.freeze())
+            .expect("semicolon separated HeaderValueStrings are valid")
+    }
+}
+
+fn parse_token(s: &str) -> Result<&str, Error> {
+    if !s.is_empty() && s.chars().all(is_tchar) {
+        Ok(s)
+    } else {
+        Err(Error::invalid())
+    }
+}
+
+// https://datatracker.ietf.org/doc/html/rfc7230#section-3.2.6
+fn is_tchar(c: char) -> bool {
+    matches!(
+        c,
+        '!' | '#' | '$' | '%' | '&' | '\'' | '*' |
+        '+' | '-' | '.' | '^' | '_' | '`' | '|' | '~' |
+        '0'..='9' | 'a'..='z' | 'A'..='Z'
+    )
+}
+
+fn parse_value(s: &str) -> Result<&str, Error> {
+    if let Some(quoted) = s.strip_prefix('"') {
+        if let Some(val) = quoted.strip_suffix('"') {
+            parse_token(val)
+        } else {
+            // Only had starting double quote
+            Err(Error::invalid())
+        }
+    } else {
+        // Not a quoted string.
+        parse_token(s)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::super::{test_decode, test_encode};
+    use super::*;
+
+    #[test]
+    fn extensions_decode() {
+        let extensions =
+            test_decode::<SecWebsocketExtensions>(&["key1; val1", "key2; val2"]).unwrap();
+        assert_eq!(extensions.0.len(), 2);
+        assert_eq!(
+            extensions.0[0],
+            WebsocketExtension::try_from("key1; val1").unwrap()
+        );
+        assert_eq!(
+            extensions.0[1],
+            WebsocketExtension::try_from("key2; val2").unwrap()
+        );
+
+        assert_eq!(test_decode::<SecWebsocketExtensions>(&[""]), None);
+    }
+
+    #[test]
+    fn extensions_decode_split() {
+        // Split each extension into separate headers
+        let extensions =
+            test_decode::<SecWebsocketExtensions>(&["key1; val1, key2; val2", "key3; val3"])
+                .unwrap();
+        assert_eq!(extensions.0.len(), 3);
+        assert_eq!(
+            extensions.0[0],
+            WebsocketExtension::try_from("key1; val1").unwrap()
+        );
+        assert_eq!(
+            extensions.0[1],
+            WebsocketExtension::try_from("key2; val2").unwrap()
+        );
+        assert_eq!(
+            extensions.0[2],
+            WebsocketExtension::try_from("key3; val3").unwrap()
+        );
+    }
+
+    #[test]
+    fn extensions_encode() {
+        let extensions =
+            SecWebsocketExtensions(vec![WebsocketExtension::from_static("foo; bar; baz=1")]);
+        let headers = test_encode(extensions);
+        let mut vals = headers.get_all(SEC_WEBSOCKET_EXTENSIONS).into_iter();
+        assert_eq!(vals.next().unwrap(), "foo; bar; baz=1");
+        assert_eq!(vals.next(), None);
+
+        let extensions = SecWebsocketExtensions(vec![]);
+        let headers = test_encode(extensions);
+        let mut vals = headers.get_all(SEC_WEBSOCKET_EXTENSIONS).into_iter();
+        assert_eq!(vals.next(), None);
+    }
+
+    #[test]
+    fn extensions_encode_combine() {
+        // Multiple extensions are combined into a single header
+        let extensions = SecWebsocketExtensions(vec![
+            WebsocketExtension::from_static("foo1; bar"),
+            WebsocketExtension::from_static("foo2; bar"),
+            WebsocketExtension::from_static("baz; quux"),
+        ]);
+        let headers = test_encode(extensions);
+        let mut vals = headers.get_all(SEC_WEBSOCKET_EXTENSIONS).into_iter();
+        assert_eq!(vals.next().unwrap(), "foo1; bar, foo2; bar, baz; quux");
+        assert_eq!(vals.next(), None);
+    }
+
+    #[test]
+    fn extensions_iter() {
+        let extensions = SecWebsocketExtensions(vec![
+            WebsocketExtension::from_static("foo; bar1; bar2=3"),
+            WebsocketExtension::from_static("baz; quux"),
+        ]);
+        assert_eq!(extensions.len(), 2);
+
+        let mut iter = extensions.iter();
+        let extension = iter.next().unwrap();
+        assert_eq!(extension.name(), "foo");
+        let mut params = extension.params();
+        assert_eq!(params.next(), Some(("bar1", None)));
+        assert_eq!(params.next(), Some(("bar2", Some("3"))));
+        assert!(params.next().is_none());
+
+        let extension = iter.next().unwrap();
+        assert_eq!(extension.name(), "baz");
+        let mut params = extension.params();
+        assert_eq!(params.next(), Some(("quux", None)));
+        assert!(params.next().is_none());
+
+        assert!(iter.next().is_none());
+    }
+
+    #[test]
+    fn extension_try_from_str_ok() {
+        let ext = WebsocketExtension::try_from("permessage-deflate").unwrap();
+        assert_eq!(ext.name(), "permessage-deflate");
+        let mut params = ext.params();
+        assert_eq!(params.next(), None);
+
+        let ext =
+            WebsocketExtension::try_from("permessage-deflate; client_max_window_bits").unwrap();
+        assert_eq!(ext.name(), "permessage-deflate");
+        let mut params = ext.params();
+        assert_eq!(params.next(), Some(("client_max_window_bits", None)));
+        assert_eq!(params.next(), None);
+
+        let ext =
+            WebsocketExtension::try_from("permessage-deflate; server_max_window_bits=10").unwrap();
+        assert_eq!(ext.name(), "permessage-deflate");
+        let mut params = ext.params();
+        assert_eq!(params.next(), Some(("server_max_window_bits", Some("10"))));
+        assert_eq!(params.next(), None);
+
+        let ext = WebsocketExtension::try_from("permessage-deflate; server_max_window_bits=\"10\"")
+            .unwrap();
+        assert_eq!(ext.name(), "permessage-deflate");
+        let mut params = ext.params();
+        assert_eq!(params.next(), Some(("server_max_window_bits", Some("10"))));
+        assert_eq!(params.next(), None);
+    }
+
+    #[test]
+    fn extension_try_from_str_err() {
+        assert!(WebsocketExtension::try_from("").is_err());
+        // Only single extension is allowed
+        assert!(WebsocketExtension::try_from("permessage-deflate, permessage-snappy").is_err());
+    }
+
+    #[test]
+    fn parse_value_err() {
+        #[rustfmt::skip]
+        let cases = [
+            // not token
+            "",
+            " ",
+            // Only starting quote
+            r#"""#,
+            r#""10"#,
+            // Multiple quotes
+            r#"""1"""#,
+            // Not a token after removing quotes
+            r#"" ""#,
+            r#"",""#,
+        ];
+        for case in cases {
+            assert!(parse_value(case).is_err());
+        }
+    }
+
+    #[test]
+    fn parse_value_ok() {
+        #[rustfmt::skip]
+        let cases = [
+            // Not quoted
+            r#"1"#,
+            r#"10"#,
+            r#"10.1"#,
+            // valid quoted-string
+            r#""9""#,
+            r#""val""#,
+        ];
+        for case in cases {
+            assert!(parse_value(case).is_ok());
+        }
+    }
+}
diff --git a/src/util/mod.rs b/src/util/mod.rs
index 07fddbfb..4ebb754c 100644
--- a/src/util/mod.rs
+++ b/src/util/mod.rs
@@ -3,7 +3,7 @@ use HeaderValue;
 //pub use self::charset::Charset;
 //pub use self::encoding::Encoding;
 pub(crate) use self::entity::{EntityTag, EntityTagRange};
-pub(crate) use self::flat_csv::{FlatCsv, SemiColon};
+pub(crate) use self::flat_csv::{Comma, FlatCsv, SemiColon};
 pub(crate) use self::fmt::fmt;
 pub(crate) use self::http_date::HttpDate;
 pub(crate) use self::iter::IterExt;
diff --git a/src/util/value_string.rs b/src/util/value_string.rs
index 865a3558..0e2d797c 100644
--- a/src/util/value_string.rs
+++ b/src/util/value_string.rs
@@ -26,6 +26,15 @@ impl HeaderValueString {
         }
     }
 
+    pub(crate) fn from_str(src: &str) -> Result<Self, ::Error> {
+        let value = HeaderValue::from_str(src).map_err(|_| ::Error::invalid())?;
+        if value.to_str().is_ok() {
+            Ok(HeaderValueString { value })
+        } else {
+            Err(::Error::invalid())
+        }
+    }
+
     pub(crate) fn from_string(src: String) -> Option<Self> {
         // A valid `str` (the argument)...
         let bytes = Bytes::from(src);