diff --git a/crates/parse/src/parser/ty.rs b/crates/parse/src/parser/ty.rs
index fde9cc1e..c928d4ba 100644
--- a/crates/parse/src/parser/ty.rs
+++ b/crates/parse/src/parser/ty.rs
@@ -114,19 +114,30 @@ impl<'sess, 'ast> Parser<'sess, 'ast> {
         Ok(ty)
     }
 
+    /// Parses a mapping key type.
+    fn parse_mapping_key_type(&mut self) -> PResult<'sess, Type<'ast>> {
+        if self.check_elementary_type() {
+            self.parse_spanned(Self::parse_elementary_type)
+                .map(|(span, kind)| Type { span, kind: TypeKind::Elementary(kind) })
+        } else if self.check_path() {
+            self.parse_spanned(Self::parse_path)
+                .map(|(span, path)| Type { span, kind: TypeKind::Custom(path) })
+        } else {
+            self.unexpected()
+        }
+    }
+
     /// Parses a mapping type.
     fn parse_mapping_type(&mut self) -> PResult<'sess, TypeMapping<'ast>> {
         self.expect(TokenKind::OpenDelim(Delimiter::Parenthesis))?;
 
-        let key = self.parse_type()?;
-        // TODO: Move to type checking.
-        if !key.is_elementary() && !key.is_custom() {
-            let msg =
-                "only elementary types or used-defined types can be used as key types in mappings";
-            self.dcx().err(msg).span(key.span).emit();
-        }
+        let key = self.parse_mapping_key_type()?;
         let key_name = self.parse_ident_opt()?;
 
+        // We are about to require a `=>`. Previous checks (e.g. state mutability on elementary
+        // types) may have added expectations like `payable`/`pure`/`view`. Clear them so the
+        // diagnostic only mentions `=>` here.
+        self.expected_tokens.clear();
         self.expect(TokenKind::FatArrow)?;
 
         let value = self.parse_type()?;
diff --git a/crates/sema/src/typeck/checker.rs b/crates/sema/src/typeck/checker.rs
index 9ca6ae0b..14a616a7 100644
--- a/crates/sema/src/typeck/checker.rs
+++ b/crates/sema/src/typeck/checker.rs
@@ -625,6 +625,18 @@ impl<'gcx> TypeChecker<'gcx> {
         // TODO: checks from https://github.com/ethereum/solidity/blob/9d7cc42bc1c12bb43e9dccf8c6c36833fdfcbbca/libsolidity/analysis/TypeChecker.cpp#L1219
     }
 
+    #[must_use]
+    fn check_mapping_key_type(&mut self, key: &'gcx hir::Type<'gcx>) -> Ty<'gcx> {
+        let ty = self.gcx.type_of_hir_ty(key);
+        if !matches!(
+            ty.kind,
+            TyKind::Elementary(_) | TyKind::Udvt(_, _) | TyKind::Contract(_) | TyKind::Enum(_)
+        ) {
+            self.dcx().err("only elementary types, user defined value types, contract types or enums are allowed as mapping keys.").span(key.span).emit();
+        }
+        ty
+    }
+
     #[must_use]
     fn require_lvalue(&mut self, expr: &'gcx hir::Expr<'gcx>) -> Ty<'gcx> {
         let prev = self.lvalue_context.replace(true);
@@ -731,7 +743,6 @@ impl<'gcx> hir::Visit<'gcx> for TypeChecker<'gcx> {
         ControlFlow::Continue(())
     }
 
-    #[expect(clippy::single_match)]
     fn visit_ty(&mut self, hir_ty: &'gcx hir::Type<'gcx>) -> ControlFlow<Self::BreakValue> {
         match hir_ty.kind {
             hir::TypeKind::Array(array) => {
@@ -740,6 +751,10 @@ impl<'gcx> hir::Visit<'gcx> for TypeChecker<'gcx> {
                 }
                 return self.visit_ty(&array.element);
             }
+            hir::TypeKind::Mapping(mapping) => {
+                let _ = self.check_mapping_key_type(&mapping.key);
+                self.visit_ty(&mapping.value)?;
+            }
             // TODO: https://github.com/ethereum/solidity/blob/9d7cc42bc1c12bb43e9dccf8c6c36833fdfcbbca/libsolidity/analysis/TypeChecker.cpp#L713
             // hir::TypeKind::Function(func) => {
             //     if func.visibility == hir::Visibility::External {
diff --git a/tests/ui/parser/mapping_key_type.sol b/tests/ui/parser/mapping_key_type.sol
new file mode 100644
index 00000000..f4286b4f
--- /dev/null
+++ b/tests/ui/parser/mapping_key_type.sol
@@ -0,0 +1,20 @@
+type U is int;
+enum E {
+    A,
+    B
+}
+
+library L{
+    enum E1 {
+        A,
+        B
+    }
+}
+
+contract C {
+    mapping(uint => uint) m0;
+    mapping(E => uint) m1;
+    mapping(U => uint) m2;
+    mapping(L.E1 => uint) m3;
+    mapping(uint[] => uint) m4; //~ ERROR: expected `=>`, found `[`
+}
diff --git a/tests/ui/parser/mapping_key_type.stderr b/tests/ui/parser/mapping_key_type.stderr
new file mode 100644
index 00000000..0f8b04fb
--- /dev/null
+++ b/tests/ui/parser/mapping_key_type.stderr
@@ -0,0 +1,8 @@
+error: expected `=>`, found `[`
+  --> ROOT/tests/ui/parser/mapping_key_type.sol:LL:CC
+   |
+LL |     mapping(uint[] => uint) m4;
+   |                 ^ expected `=>`
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/typeck/mapping_key_type_check.sol b/tests/ui/typeck/mapping_key_type_check.sol
new file mode 100644
index 00000000..9dd7d1ab
--- /dev/null
+++ b/tests/ui/typeck/mapping_key_type_check.sol
@@ -0,0 +1,25 @@
+//@compile-flags: -Ztypeck
+type U is int;
+enum E {
+    A,
+    B
+}
+
+library L{
+    enum E1 {
+        A,
+        B
+    }
+    struct S1 {
+        int x;
+    }
+}
+
+contract C {
+    mapping(uint => uint) m0;
+    mapping(string => uint) m1;
+    mapping(E => uint) m2;
+    mapping(U => uint) m3;
+    mapping(L.E1 => uint) m4;
+    mapping(L.S1 => uint) m5; //~ ERROR: only elementary types, user defined value types, contract types or enums are allowed as mapping keys.
+}
diff --git a/tests/ui/typeck/mapping_key_type_check.stderr b/tests/ui/typeck/mapping_key_type_check.stderr
new file mode 100644
index 00000000..24f3df6e
--- /dev/null
+++ b/tests/ui/typeck/mapping_key_type_check.stderr
@@ -0,0 +1,8 @@
+error: only elementary types, user defined value types, contract types or enums are allowed as mapping keys.
+  --> ROOT/tests/ui/typeck/mapping_key_type_check.sol:LL:CC
+   |
+LL |     mapping(L.S1 => uint) m5;
+   |             ^^^^
+
+error: aborting due to 1 previous error
+