diff --git a/src/lints/unconditionally_sealed_trait_became_unsealed.ron b/src/lints/unconditionally_sealed_trait_became_unsealed.ron
new file mode 100644
index 00000000..7f3fdd7d
--- /dev/null
+++ b/src/lints/unconditionally_sealed_trait_became_unsealed.ron
@@ -0,0 +1,54 @@
+SemverQuery(
+    id: "unconditionally_sealed_trait_became_unsealed",
+    human_readable_name: "unconditionally sealed trait became unsealed",
+    description: "An unconditionally 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: ["$public"]) @output
+                        unconditionally_sealed @filter(op: "=", value: ["$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: ["$true"])
+                        public_api_sealed @filter(op: "!=", value: ["$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: "An unconditionally 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 b5bab28d..378bb7dc 100644
--- a/src/query.rs
+++ b/src/query.rs
@@ -1355,6 +1355,7 @@ add_lints!(
     type_mismatched_generic_lifetimes,
     type_requires_more_const_generic_params,
     type_requires_more_generic_type_params,
+    unconditionally_sealed_trait_became_unsealed,
     union_field_added_with_all_pub_fields,
     union_field_added_with_non_pub_fields,
     union_field_missing,
diff --git a/test_crates/unconditionally_sealed_trait_became_unsealed/new/Cargo.toml b/test_crates/unconditionally_sealed_trait_became_unsealed/new/Cargo.toml
new file mode 100644
index 00000000..f46baa74
--- /dev/null
+++ b/test_crates/unconditionally_sealed_trait_became_unsealed/new/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+publish = false
+name = "unconditionally_sealed_trait_became_unsealed"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
diff --git a/test_crates/unconditionally_sealed_trait_became_unsealed/new/src/lib.rs b/test_crates/unconditionally_sealed_trait_became_unsealed/new/src/lib.rs
new file mode 100644
index 00000000..8b92e78c
--- /dev/null
+++ b/test_crates/unconditionally_sealed_trait_became_unsealed/new/src/lib.rs
@@ -0,0 +1,40 @@
+// Traits transitioning from Unconditionally Sealed → Unsealed (Lint should detect these)
+pub mod unconditionally_sealed_to_unsealed {
+    pub mod hidden {
+        pub trait Sealed {}
+        pub struct Token;
+    }
+
+    pub trait ExtendsTraitInHiddenModule: hidden::Sealed {}
+
+    pub trait TransitivelyTraitSealed: ExtendsTraitInHiddenModule {}
+
+    pub trait ReturnsTraitInHiddenModule {
+        fn method(&self) -> hidden::Token;
+    }
+
+    pub trait AcceptsTraitInHiddenModule {
+        fn method(&self, token: hidden::Token);
+    }
+
+    pub trait SealedWithWhereSelfBound
+    where
+        Self: hidden::Sealed,
+    {
+    }
+}
+
+// Traits transitioning from Unconditionally Sealed -> Public API Sealed (Lint should not detect these)
+pub mod unconditionally_sealed_to_public_api_sealed {
+    #[doc(hidden)]
+    pub mod hidden {
+        pub trait Sealed {}
+        pub struct Token;
+    }
+
+    pub trait ExtendsTraitInHiddenModule: hidden::Sealed {}
+
+    pub trait ReturnsTraitInHiddenModule {
+        fn method(&self) -> hidden::Token;
+    }
+}
diff --git a/test_crates/unconditionally_sealed_trait_became_unsealed/old/Cargo.toml b/test_crates/unconditionally_sealed_trait_became_unsealed/old/Cargo.toml
new file mode 100644
index 00000000..f46baa74
--- /dev/null
+++ b/test_crates/unconditionally_sealed_trait_became_unsealed/old/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+publish = false
+name = "unconditionally_sealed_trait_became_unsealed"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
diff --git a/test_crates/unconditionally_sealed_trait_became_unsealed/old/src/lib.rs b/test_crates/unconditionally_sealed_trait_became_unsealed/old/src/lib.rs
new file mode 100644
index 00000000..f0584834
--- /dev/null
+++ b/test_crates/unconditionally_sealed_trait_became_unsealed/old/src/lib.rs
@@ -0,0 +1,39 @@
+// Traits transitioning from Unconditionally Sealed → Unsealed (Lint should detect these)
+pub mod unconditionally_sealed_to_unsealed {
+    mod hidden {
+        pub trait Sealed {}
+        pub struct Token;
+    }
+
+    pub trait ExtendsTraitInHiddenModule: hidden::Sealed {}
+
+    pub trait TransitivelyTraitSealed: ExtendsTraitInHiddenModule {}
+
+    pub trait ReturnsTraitInHiddenModule {
+        fn method(&self) -> hidden::Token;
+    }
+
+    pub trait AcceptsTraitInHiddenModule {
+        fn method(&self, token: hidden::Token);
+    }
+
+    pub trait SealedWithWhereSelfBound
+    where
+        Self: hidden::Sealed,
+    {
+    }
+}
+
+// Traits transitioning from Unconditionally Sealed -> Public API Sealed (Lint should not detect these)
+pub mod unconditionally_sealed_to_public_api_sealed {
+    mod hidden {
+        pub trait Sealed {}
+        pub struct Token;
+    }
+
+    pub trait ExtendsTraitInHiddenModule: hidden::Sealed {}
+
+    pub trait ReturnsTraitInHiddenModule {
+        fn method(&self) -> hidden::Token;
+    }
+}
diff --git a/test_outputs/query_execution/unconditionally_sealed_trait_became_unsealed.snap b/test_outputs/query_execution/unconditionally_sealed_trait_became_unsealed.snap
new file mode 100644
index 00000000..2844716f
--- /dev/null
+++ b/test_outputs/query_execution/unconditionally_sealed_trait_became_unsealed.snap
@@ -0,0 +1,81 @@
+---
+source: src/query.rs
+expression: "&query_execution_results"
+---
+{
+  "./test_crates/trait_method_added/": [
+    {
+      "name": String("WillGainMethodWithoutDefaultAndLoseSeal"),
+      "path": List([
+        String("trait_method_added"),
+        String("WillGainMethodWithoutDefaultAndLoseSeal"),
+      ]),
+      "span_begin_line": Uint64(43),
+      "span_end_line": Uint64(45),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+  ],
+  "./test_crates/unconditionally_sealed_trait_became_unsealed/": [
+    {
+      "name": String("ExtendsTraitInHiddenModule"),
+      "path": List([
+        String("unconditionally_sealed_trait_became_unsealed"),
+        String("unconditionally_sealed_to_unsealed"),
+        String("ExtendsTraitInHiddenModule"),
+      ]),
+      "span_begin_line": Uint64(8),
+      "span_end_line": Uint64(8),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("TransitivelyTraitSealed"),
+      "path": List([
+        String("unconditionally_sealed_trait_became_unsealed"),
+        String("unconditionally_sealed_to_unsealed"),
+        String("TransitivelyTraitSealed"),
+      ]),
+      "span_begin_line": Uint64(10),
+      "span_end_line": Uint64(10),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("ReturnsTraitInHiddenModule"),
+      "path": List([
+        String("unconditionally_sealed_trait_became_unsealed"),
+        String("unconditionally_sealed_to_unsealed"),
+        String("ReturnsTraitInHiddenModule"),
+      ]),
+      "span_begin_line": Uint64(12),
+      "span_end_line": Uint64(14),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("AcceptsTraitInHiddenModule"),
+      "path": List([
+        String("unconditionally_sealed_trait_became_unsealed"),
+        String("unconditionally_sealed_to_unsealed"),
+        String("AcceptsTraitInHiddenModule"),
+      ]),
+      "span_begin_line": Uint64(16),
+      "span_end_line": Uint64(18),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("SealedWithWhereSelfBound"),
+      "path": List([
+        String("unconditionally_sealed_trait_became_unsealed"),
+        String("unconditionally_sealed_to_unsealed"),
+        String("SealedWithWhereSelfBound"),
+      ]),
+      "span_begin_line": Uint64(20),
+      "span_end_line": Uint64(24),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+  ],
+}