diff --git a/src/lints/pub_api_sealed_trait_became_unsealed.ron b/src/lints/pub_api_sealed_trait_became_unsealed.ron
new file mode 100644
index 00000000..61242b3b
--- /dev/null
+++ b/src/lints/pub_api_sealed_trait_became_unsealed.ron
@@ -0,0 +1,54 @@
+SemverQuery(
+    id: "pub_api_sealed_trait_became_unsealed",
+    human_readable_name: "public API sealed trait became unsealed",
+    description: "A public API sealed trait has become unsealed, allowing downstream crates to implement it. Reverting this would be a major breaking change.",
+    required_update: Minor,
+    lint_level: Warn,
+    reference_link: Some("https://rust-lang.github.io/api-guidelines/future-proofing.html#sealed-traits-protect-against-downstream-implementations-c-sealed"),
+    query: r#"
+    {
+        CrateDiff {
+            baseline {
+                item {
+                    ... on Trait {
+                        visibility_limit @filter(op: "=", value: ["$public"]) @output
+                        public_api_sealed @filter(op: "=", value: ["$true"])
+                        unconditionally_sealed @filter(op: "!=", value: ["$true"])
+
+                        importable_path {
+                            path @output @tag
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+                    }
+                }
+            }
+            current {
+                item {
+                    ... on Trait {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+                        public_api_sealed @filter(op: "!=", value: ["$true"])
+                        name @output
+
+                        importable_path {
+                            path @filter(op: "=", value: ["%path"])
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        span_: span @optional {
+                            filename @output
+                            begin_line @output
+                            end_line @output
+                        }
+                    }
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "public": "public",
+        "true": true,
+    },
+    error_message: "A public API sealed trait has become unsealed, allowing downstream crates to implement it. Reverting this would be a major breaking change.",
+    per_result_error_template: Some("trait {{join \"::\" path}} in file {{span_filename}}:{{span_begin_line}}"),
+    witness: None,
+)
diff --git a/src/query.rs b/src/query.rs
index 1e8026ac..396bc79e 100644
--- a/src/query.rs
+++ b/src/query.rs
@@ -1340,6 +1340,7 @@ add_lints!(
     trait_removed_supertrait,
     trait_requires_more_const_generic_params,
     trait_requires_more_generic_type_params,
+    pub_api_sealed_trait_became_unsealed,
     trait_unsafe_added,
     trait_unsafe_removed,
     tuple_struct_to_plain_struct,
diff --git a/test_crates/pub_api_sealed_trait_became_unsealed/new/Cargo.toml b/test_crates/pub_api_sealed_trait_became_unsealed/new/Cargo.toml
new file mode 100644
index 00000000..8a7a0dca
--- /dev/null
+++ b/test_crates/pub_api_sealed_trait_became_unsealed/new/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+publish = false
+name = "pub_api_sealed_trait_became_unsealed"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
diff --git a/test_crates/pub_api_sealed_trait_became_unsealed/new/src/lib.rs b/test_crates/pub_api_sealed_trait_became_unsealed/new/src/lib.rs
new file mode 100644
index 00000000..c3ac8d82
--- /dev/null
+++ b/test_crates/pub_api_sealed_trait_became_unsealed/new/src/lib.rs
@@ -0,0 +1,39 @@
+// Traits transitioning from Public API Sealed → Unsealed (Lint should detect these)
+pub mod public_api_sealed_to_unsealed {
+    pub mod hidden {
+        pub trait Sealed {}
+        pub struct Token;
+    }
+
+    pub trait PublicAPIToBeUnsealed {
+        type Hidden;
+    }
+
+    pub trait TraitExtendsHiddenPublicAPITrait: hidden::Sealed {}
+
+    pub trait MethodReturnPublicAPIHiddenToken {
+        fn method(&self) -> hidden::Token;
+    }
+
+    pub trait MethodTakingPublicAPIHiddenToken {
+        fn method(&self, token: hidden::Token);
+    }
+}
+
+// Traits transitioning from Public API Sealed → Unconditionally Sealed (Lint should ignore these)
+pub mod public_api_sealed_to_unconditionally_sealed {
+    mod hidden {
+        pub trait Sealed {}
+        pub struct Token;
+    }
+
+    pub trait TraitExtendsUnconditionallyHiddenTrait: hidden::Sealed {}
+
+    pub trait MethodReturningUnconditionallyHiddenToken {
+        fn method(&self) -> hidden::Token;
+    }
+
+    pub trait MethodTakingUnconditionallyHiddenToken {
+        fn method(&self, token: hidden::Token);
+    }
+}
diff --git a/test_crates/pub_api_sealed_trait_became_unsealed/old/Cargo.toml b/test_crates/pub_api_sealed_trait_became_unsealed/old/Cargo.toml
new file mode 100644
index 00000000..8a7a0dca
--- /dev/null
+++ b/test_crates/pub_api_sealed_trait_became_unsealed/old/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+publish = false
+name = "pub_api_sealed_trait_became_unsealed"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
diff --git a/test_crates/pub_api_sealed_trait_became_unsealed/old/src/lib.rs b/test_crates/pub_api_sealed_trait_became_unsealed/old/src/lib.rs
new file mode 100644
index 00000000..c8506d29
--- /dev/null
+++ b/test_crates/pub_api_sealed_trait_became_unsealed/old/src/lib.rs
@@ -0,0 +1,42 @@
+// Traits transitioning from Public API Sealed → Unsealed (Lint should detect these)
+pub mod public_api_sealed_to_unsealed {
+    #[doc(hidden)]
+    pub mod hidden {
+        pub trait Sealed {}
+        pub struct Token;
+    }
+
+    pub trait PublicAPIToBeUnsealed {
+        #[doc(hidden)]
+        type Hidden;
+    }
+
+    pub trait TraitExtendsHiddenPublicAPITrait: hidden::Sealed {}
+
+    pub trait MethodReturnPublicAPIHiddenToken {
+        fn method(&self) -> hidden::Token;
+    }
+
+    pub trait MethodTakingPublicAPIHiddenToken {
+        fn method(&self, token: hidden::Token);
+    }
+}
+
+// Traits transitioning from Public API Sealed → Unconditionally Sealed (Lint should ignore these)
+pub mod public_api_sealed_to_unconditionally_sealed {
+    #[doc(hidden)]
+    pub mod hidden {
+        pub trait Sealed {}
+        pub struct Token;
+    }
+
+    pub trait TraitExtendsUnconditionallyHiddenTrait: hidden::Sealed {}
+
+    pub trait MethodReturningUnconditionallyHiddenToken {
+        fn method(&self) -> hidden::Token;
+    }
+
+    pub trait MethodTakingUnconditionallyHiddenToken {
+        fn method(&self, token: hidden::Token);
+    }
+}
diff --git a/test_outputs/query_execution/pub_api_sealed_trait_became_unsealed.snap b/test_outputs/query_execution/pub_api_sealed_trait_became_unsealed.snap
new file mode 100644
index 00000000..fe591aed
--- /dev/null
+++ b/test_outputs/query_execution/pub_api_sealed_trait_became_unsealed.snap
@@ -0,0 +1,82 @@
+---
+source: src/query.rs
+expression: "&query_execution_results"
+---
+{
+  "./test_crates/pub_api_sealed_trait_became_unsealed/": [
+    {
+      "name": String("PublicAPIToBeUnsealed"),
+      "path": List([
+        String("pub_api_sealed_trait_became_unsealed"),
+        String("public_api_sealed_to_unsealed"),
+        String("PublicAPIToBeUnsealed"),
+      ]),
+      "span_begin_line": Uint64(8),
+      "span_end_line": Uint64(10),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("TraitExtendsHiddenPublicAPITrait"),
+      "path": List([
+        String("pub_api_sealed_trait_became_unsealed"),
+        String("public_api_sealed_to_unsealed"),
+        String("TraitExtendsHiddenPublicAPITrait"),
+      ]),
+      "span_begin_line": Uint64(12),
+      "span_end_line": Uint64(12),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("MethodReturnPublicAPIHiddenToken"),
+      "path": List([
+        String("pub_api_sealed_trait_became_unsealed"),
+        String("public_api_sealed_to_unsealed"),
+        String("MethodReturnPublicAPIHiddenToken"),
+      ]),
+      "span_begin_line": Uint64(14),
+      "span_end_line": Uint64(16),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("MethodTakingPublicAPIHiddenToken"),
+      "path": List([
+        String("pub_api_sealed_trait_became_unsealed"),
+        String("public_api_sealed_to_unsealed"),
+        String("MethodTakingPublicAPIHiddenToken"),
+      ]),
+      "span_begin_line": Uint64(18),
+      "span_end_line": Uint64(20),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+  ],
+  "./test_crates/trait_associated_type_marked_deprecated/": [
+    {
+      "name": String("PublicTraitWithHiddenType"),
+      "path": List([
+        String("trait_associated_type_marked_deprecated"),
+        String("PublicTraitWithHiddenType"),
+      ]),
+      "span_begin_line": Uint64(40),
+      "span_end_line": Uint64(44),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+  ],
+  "./test_crates/trait_method_marked_deprecated/": [
+    {
+      "name": String("PublicTraitWithHiddenMethod"),
+      "path": List([
+        String("trait_method_marked_deprecated"),
+        String("PublicTraitWithHiddenMethod"),
+      ]),
+      "span_begin_line": Uint64(47),
+      "span_end_line": Uint64(51),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+  ],
+}