diff --git a/src/lints/pub_api_sealed_trait_became_unconditionally_sealed.ron b/src/lints/pub_api_sealed_trait_became_unconditionally_sealed.ron
new file mode 100644
index 00000000..c975be3e
--- /dev/null
+++ b/src/lints/pub_api_sealed_trait_became_unconditionally_sealed.ron
@@ -0,0 +1,54 @@
+SemverQuery(
+    id: "pub_api_sealed_trait_became_unconditionally_sealed",
+    human_readable_name: "public API sealed trait became unconditionally sealed",
+    description: "A public API sealed trait has become unconditionally sealed, blocking all downstream implementations including those from first-party crates that rely on the non-public API.",
+    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: ["$public"])
+                        unconditionally_sealed @filter(op: "=", value: ["$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 unconditionally sealed, blocking all downstream implementations including those from first-party crates that rely on the non-public API.",
+    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 378bb7dc..795d3679 100644
--- a/src/query.rs
+++ b/src/query.rs
@@ -1287,6 +1287,7 @@ add_lints!(
     partial_ord_struct_fields_reordered,
     proc_macro_marked_deprecated,
     proc_macro_now_doc_hidden,
+    pub_api_sealed_trait_became_unconditionally_sealed,
     pub_module_level_const_missing,
     pub_module_level_const_now_doc_hidden,
     pub_static_missing,
diff --git a/test_crates/pub_api_sealed_trait_became_unconditionally_sealed/new/Cargo.toml b/test_crates/pub_api_sealed_trait_became_unconditionally_sealed/new/Cargo.toml
new file mode 100644
index 00000000..f1dcd7f3
--- /dev/null
+++ b/test_crates/pub_api_sealed_trait_became_unconditionally_sealed/new/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+publish = false
+name = "pub_api_sealed_trait_became_unconditionally_sealed"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
diff --git a/test_crates/pub_api_sealed_trait_became_unconditionally_sealed/new/src/lib.rs b/test_crates/pub_api_sealed_trait_became_unconditionally_sealed/new/src/lib.rs
new file mode 100644
index 00000000..818e3004
--- /dev/null
+++ b/test_crates/pub_api_sealed_trait_became_unconditionally_sealed/new/src/lib.rs
@@ -0,0 +1,23 @@
+// Traits transitioning from Public API Sealed → Unconditionally Sealed (Lint should detect 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);
+    }
+
+    pub trait HiddenSealedWithWhereSelfBound
+    where
+        Self: hidden::Sealed,
+    {
+    }
+}
diff --git a/test_crates/pub_api_sealed_trait_became_unconditionally_sealed/old/Cargo.toml b/test_crates/pub_api_sealed_trait_became_unconditionally_sealed/old/Cargo.toml
new file mode 100644
index 00000000..f1dcd7f3
--- /dev/null
+++ b/test_crates/pub_api_sealed_trait_became_unconditionally_sealed/old/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+publish = false
+name = "pub_api_sealed_trait_became_unconditionally_sealed"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
diff --git a/test_crates/pub_api_sealed_trait_became_unconditionally_sealed/old/src/lib.rs b/test_crates/pub_api_sealed_trait_became_unconditionally_sealed/old/src/lib.rs
new file mode 100644
index 00000000..6f4e07b9
--- /dev/null
+++ b/test_crates/pub_api_sealed_trait_became_unconditionally_sealed/old/src/lib.rs
@@ -0,0 +1,24 @@
+// Traits transitioning from Public API Sealed → Unconditionally Sealed (Lint should detect 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);
+    }
+
+    pub trait HiddenSealedWithWhereSelfBound
+    where
+        Self: hidden::Sealed,
+    {
+    }
+}
diff --git a/test_outputs/query_execution/pub_api_sealed_trait_became_unconditionally_sealed.snap b/test_outputs/query_execution/pub_api_sealed_trait_became_unconditionally_sealed.snap
new file mode 100644
index 00000000..c6878108
--- /dev/null
+++ b/test_outputs/query_execution/pub_api_sealed_trait_became_unconditionally_sealed.snap
@@ -0,0 +1,107 @@
+---
+source: src/query.rs
+expression: "&query_execution_results"
+---
+{
+  "./test_crates/pub_api_sealed_trait_became_unconditionally_sealed/": [
+    {
+      "name": String("TraitExtendsUnconditionallyHiddenTrait"),
+      "path": List([
+        String("pub_api_sealed_trait_became_unconditionally_sealed"),
+        String("public_api_sealed_to_unconditionally_sealed"),
+        String("TraitExtendsUnconditionallyHiddenTrait"),
+      ]),
+      "span_begin_line": Uint64(8),
+      "span_end_line": Uint64(8),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("MethodReturningUnconditionallyHiddenToken"),
+      "path": List([
+        String("pub_api_sealed_trait_became_unconditionally_sealed"),
+        String("public_api_sealed_to_unconditionally_sealed"),
+        String("MethodReturningUnconditionallyHiddenToken"),
+      ]),
+      "span_begin_line": Uint64(10),
+      "span_end_line": Uint64(12),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("MethodTakingUnconditionallyHiddenToken"),
+      "path": List([
+        String("pub_api_sealed_trait_became_unconditionally_sealed"),
+        String("public_api_sealed_to_unconditionally_sealed"),
+        String("MethodTakingUnconditionallyHiddenToken"),
+      ]),
+      "span_begin_line": Uint64(14),
+      "span_end_line": Uint64(16),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("HiddenSealedWithWhereSelfBound"),
+      "path": List([
+        String("pub_api_sealed_trait_became_unconditionally_sealed"),
+        String("public_api_sealed_to_unconditionally_sealed"),
+        String("HiddenSealedWithWhereSelfBound"),
+      ]),
+      "span_begin_line": Uint64(18),
+      "span_end_line": Uint64(22),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+  ],
+  "./test_crates/pub_api_sealed_trait_became_unsealed/": [
+    {
+      "name": String("TraitExtendsUnconditionallyHiddenTrait"),
+      "path": List([
+        String("pub_api_sealed_trait_became_unsealed"),
+        String("public_api_sealed_to_unconditionally_sealed"),
+        String("TraitExtendsUnconditionallyHiddenTrait"),
+      ]),
+      "span_begin_line": Uint64(30),
+      "span_end_line": Uint64(30),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("MethodReturningUnconditionallyHiddenToken"),
+      "path": List([
+        String("pub_api_sealed_trait_became_unsealed"),
+        String("public_api_sealed_to_unconditionally_sealed"),
+        String("MethodReturningUnconditionallyHiddenToken"),
+      ]),
+      "span_begin_line": Uint64(32),
+      "span_end_line": Uint64(34),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("MethodTakingUnconditionallyHiddenToken"),
+      "path": List([
+        String("pub_api_sealed_trait_became_unsealed"),
+        String("public_api_sealed_to_unconditionally_sealed"),
+        String("MethodTakingUnconditionallyHiddenToken"),
+      ]),
+      "span_begin_line": Uint64(36),
+      "span_end_line": Uint64(38),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+  ],
+  "./test_crates/trait_newly_sealed/": [
+    {
+      "name": String("PublicAPISealed"),
+      "path": List([
+        String("trait_newly_sealed"),
+        String("PublicAPISealed"),
+      ]),
+      "span_begin_line": Uint64(29),
+      "span_end_line": Uint64(32),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+  ],
+}