diff --git a/src/lints/trait_associated_type_added.ron b/src/lints/trait_associated_type_added.ron
new file mode 100644
index 00000000..54a41235
--- /dev/null
+++ b/src/lints/trait_associated_type_added.ron
@@ -0,0 +1,59 @@
+SemverQuery(
+    id: "trait_associated_type_added",
+    human_readable_name: "non-sealed public trait added associated type without default value",
+    description: "A non-sealed trait has gained an associated type without a default value, which breaks downstream implementations of the trait",
+    required_update: Major,
+    lint_level: Deny,
+    reference_link: Some("https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-item-no-default"),
+    query: r#"
+    {
+        CrateDiff {
+            current {
+                item {
+                    ... on Trait {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+
+                        importable_path {
+                            path @output @tag
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        associated_type {
+                            associated_type: name @output @tag
+                            has_default @filter(op: "!=", value: ["$true"]) @output
+
+                            span_: span @optional {
+                               filename @output
+                               begin_line @output
+                            }
+                        }
+                    }
+                }
+            }
+            baseline {
+                item {
+                    ... on Trait {
+                        visibility_limit @filter(op: "=", value: ["$public"]) @output
+                        sealed @filter(op: "!=", value: ["$true"])
+
+                        importable_path {
+                            path @filter(op: "=", value: ["%path"])
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        associated_type @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                            name @filter(op: "=", value: ["%associated_type"])
+                        }
+                    }
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "public": "public",
+        "true": true,
+        "zero": 0,
+    },
+    error_message: "A non-sealed trait has gained an associated type without a default value, which breaks downstream implementations of the trait",
+    per_result_error_template: Some("trait associated type {{join \"::\" path}}::{{associated_type}} in file {{span_filename}}:{{span_begin_line}}"),
+)
diff --git a/src/query.rs b/src/query.rs
index 58c3b420..16bfdbe0 100644
--- a/src/query.rs
+++ b/src/query.rs
@@ -836,6 +836,7 @@ add_lints!(
     trait_associated_const_added,
     trait_associated_const_default_removed,
     trait_associated_const_now_doc_hidden,
+    trait_associated_type_added,
     trait_associated_type_now_doc_hidden,
     trait_default_impl_removed,
     trait_method_missing,
diff --git a/test_crates/trait_associated_type_added/new/Cargo.toml b/test_crates/trait_associated_type_added/new/Cargo.toml
new file mode 100644
index 00000000..08fc1a6f
--- /dev/null
+++ b/test_crates/trait_associated_type_added/new/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+publish = false
+name = "trait_associated_type_added"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
diff --git a/test_crates/trait_associated_type_added/new/src/lib.rs b/test_crates/trait_associated_type_added/new/src/lib.rs
new file mode 100644
index 00000000..c0167839
--- /dev/null
+++ b/test_crates/trait_associated_type_added/new/src/lib.rs
@@ -0,0 +1,69 @@
+#![feature(associated_type_defaults)]
+
+mod sealed {
+    pub(crate) trait Sealed {}
+}
+
+pub trait WillGainTypeWithoutDefault {
+    type Bar;
+
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait WillGainTypeWithoutDefaultSealed: sealed::Sealed {
+    type Bar;
+
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait WillGainTypeWithoutDefaultAndSeal: sealed::Sealed {
+    type Bar;
+
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait WillGainTypeWithDefault {
+    type Bar = bool;
+
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait WillGainTypeWithDefaultSealed: sealed::Sealed {
+    type Bar = bool;
+
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait WillGainAnotherTypeWithoutDefault {
+    type One;
+    type Two;
+
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait WillGainAnotherTypeWithoutDefaultSealed: sealed::Sealed {
+    type One;
+    type Two;
+
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait WillGainADocHiddenType {
+    #[doc(hidden)]
+    type Bar;
+
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait TypeWithoutDefaultUnchanged {
+    type Bar;
+
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait TypeDocHidden {
+    #[doc(hidden)]
+    type Bar;
+
+    fn make_me_non_object_safe() -> Self;
+}
diff --git a/test_crates/trait_associated_type_added/old/Cargo.toml b/test_crates/trait_associated_type_added/old/Cargo.toml
new file mode 100644
index 00000000..08fc1a6f
--- /dev/null
+++ b/test_crates/trait_associated_type_added/old/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+publish = false
+name = "trait_associated_type_added"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
diff --git a/test_crates/trait_associated_type_added/old/src/lib.rs b/test_crates/trait_associated_type_added/old/src/lib.rs
new file mode 100644
index 00000000..70ce4787
--- /dev/null
+++ b/test_crates/trait_associated_type_added/old/src/lib.rs
@@ -0,0 +1,53 @@
+#![feature(associated_type_defaults)]
+
+mod sealed {
+    pub(crate) trait Sealed {}
+}
+
+pub trait WillGainTypeWithoutDefault {
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait WillGainTypeWithoutDefaultSealed: sealed::Sealed {
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait WillGainTypeWithoutDefaultAndSeal {
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait WillGainTypeWithDefault {
+    fn make_me_non_object_safe() -> Self;
+}
+pub trait WillGainTypeWithDefaultSealed: sealed::Sealed {
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait WillGainAnotherTypeWithoutDefault {
+    type One;
+
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait WillGainAnotherTypeWithoutDefaultSealed: sealed::Sealed {
+    type One;
+
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait WillGainADocHiddenType {
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait TypeWithoutDefaultUnchanged {
+    type Bar;
+
+    fn make_me_non_object_safe() -> Self;
+}
+
+pub trait TypeDocHidden {
+    #[doc(hidden)]
+    type Bar;
+
+    fn make_me_non_object_safe() -> Self;
+}
diff --git a/test_outputs/trait_associated_type_added.output.ron b/test_outputs/trait_associated_type_added.output.ron
new file mode 100644
index 00000000..76aeab57
--- /dev/null
+++ b/test_outputs/trait_associated_type_added.output.ron
@@ -0,0 +1,48 @@
+{
+    "./test_crates/trait_associated_type_added/": [
+        {
+            "associated_type": String("Bar"),
+            "has_default": Boolean(false),
+            "path": List([
+                String("trait_associated_type_added"),
+                String("WillGainTypeWithoutDefault"),
+            ]),
+            "span_begin_line": Uint64(8),
+            "span_filename": String("src/lib.rs"),
+            "visibility_limit": String("public"),
+        },
+        {
+            "associated_type": String("Bar"),
+            "has_default": Boolean(false),
+            "path": List([
+                String("trait_associated_type_added"),
+                String("WillGainTypeWithoutDefaultAndSeal"),
+            ]),
+            "span_begin_line": Uint64(20),
+            "span_filename": String("src/lib.rs"),
+            "visibility_limit": String("public"),
+        },
+        {
+            "associated_type": String("Two"),
+            "has_default": Boolean(false),
+            "path": List([
+                String("trait_associated_type_added"),
+                String("WillGainAnotherTypeWithoutDefault"),
+            ]),
+            "span_begin_line": Uint64(39),
+            "span_filename": String("src/lib.rs"),
+            "visibility_limit": String("public"),
+        },
+        {
+            "associated_type": String("Bar"),
+            "has_default": Boolean(false),
+            "path": List([
+                String("trait_associated_type_added"),
+                String("WillGainADocHiddenType"),
+            ]),
+            "span_begin_line": Uint64(53),
+            "span_filename": String("src/lib.rs"),
+            "visibility_limit": String("public"),
+        },
+    ],
+}
diff --git a/test_outputs/trait_newly_sealed.output.ron b/test_outputs/trait_newly_sealed.output.ron
index 44d7fdd6..1363a9d1 100644
--- a/test_outputs/trait_newly_sealed.output.ron
+++ b/test_outputs/trait_newly_sealed.output.ron
@@ -33,6 +33,18 @@
             "visibility_limit": String("public"),
         },
     ],
+    "./test_crates/trait_associated_type_added/": [
+        {
+            "name": String("WillGainTypeWithoutDefaultAndSeal"),
+            "path": List([
+                String("trait_associated_type_added"),
+                String("WillGainTypeWithoutDefaultAndSeal"),
+            ]),
+            "span_begin_line": Uint64(19),
+            "span_filename": String("src/lib.rs"),
+            "visibility_limit": String("public"),
+        },
+    ],
     "./test_crates/trait_default_impl_removed/": [
         {
             "name": String("TraitE"),