diff --git a/src/github.rs b/src/github.rs
index d6a92688..e878a1d7 100644
--- a/src/github.rs
+++ b/src/github.rs
@@ -416,6 +416,18 @@ pub enum ReportedContentClassifiers {
     Spam,
 }
 
+#[derive(Debug, serde::Deserialize, serde::Serialize, Eq, PartialEq)]
+pub enum LockReason {
+    #[serde(rename = "off-topic")]
+    OffTopic,
+    #[serde(rename = "too heated")]
+    TooHeated,
+    #[serde(rename = "resolved")]
+    Resolved,
+    #[serde(rename = "spam")]
+    Spam,
+}
+
 #[derive(Debug, serde::Deserialize, Eq, PartialEq)]
 #[serde(rename_all = "snake_case")]
 pub enum PullRequestReviewState {
@@ -855,6 +867,36 @@ impl Issue {
         Ok(())
     }
 
+    /// Lock an issue with an optional reason.
+    pub async fn lock(
+        &self,
+        client: &GithubClient,
+        reason: Option<LockReason>,
+    ) -> anyhow::Result<()> {
+        let lock_url = format!(
+            "{}/issues/{}/lock",
+            self.repository().url(client),
+            self.number
+        );
+        #[derive(serde::Serialize)]
+        struct LockReasonIssue {
+            lock_reason: LockReason,
+        }
+        client
+            .send_req({
+                let req = client.put(&lock_url);
+
+                if let Some(lock_reason) = reason {
+                    req.json(&LockReasonIssue { lock_reason })
+                } else {
+                    req
+                }
+            })
+            .await
+            .context("failed to lock issue")?;
+        Ok(())
+    }
+
     pub async fn close(&self, client: &GithubClient) -> anyhow::Result<()> {
         let edit_url = format!("{}/issues/{}", self.repository().url(client), self.number);
         #[derive(serde::Serialize)]
diff --git a/src/handlers/major_change.rs b/src/handlers/major_change.rs
index 0678ea42..834bda4c 100644
--- a/src/handlers/major_change.rs
+++ b/src/handlers/major_change.rs
@@ -294,6 +294,10 @@ async fn handle(
             .post_comment(&ctx.github, &comment)
             .await
             .context("post major change comment")?;
+        issue
+            .lock(&ctx.github, None)
+            .await
+            .context("lock major change issue")?;
     }
 
     let zulip_req = zulip_req.send(&ctx.github.raw());