diff --git a/lib/steep/ast/types/factory.rb b/lib/steep/ast/types/factory.rb
index 9b133c02a..20aeccca7 100644
--- a/lib/steep/ast/types/factory.rb
+++ b/lib/steep/ast/types/factory.rb
@@ -36,6 +36,29 @@ def type_1_opt(type)
           end
         end
 
+        def normalize_args(type_name, args)
+          case
+          when type_name.class?
+            if entry = env.normalized_module_class_entry(type_name)
+              type_params = entry.type_params
+            end
+          when type_name.interface?
+            if entry = env.interface_decls.fetch(type_name, nil)
+              type_params = entry.decl.type_params
+            end
+          when type_name.alias?
+            if entry = env.type_alias_decls.fetch(type_name, nil)
+              type_params = entry.decl.type_params
+            end
+          end
+
+          if type_params && !type_params.empty?
+            RBS::AST::TypeParam.normalize_args(type_params, args)
+          else
+            args
+          end
+        end
+
         def type(type)
           if ty = type_cache[type]
             return ty
@@ -68,15 +91,15 @@ def type(type)
               Name::Singleton.new(name: type_name)
             when RBS::Types::ClassInstance
               type_name = type.name
-              args = type.args.map {|arg| type(arg) }
+              args = normalize_args(type_name, type.args).map {|arg| type(arg) }
               Name::Instance.new(name: type_name, args: args)
             when RBS::Types::Interface
               type_name = type.name
-              args = type.args.map {|arg| type(arg) }
+              args = normalize_args(type_name, type.args).map {|arg| type(arg) }
               Name::Interface.new(name: type_name, args: args)
             when RBS::Types::Alias
               type_name = type.name
-              args = type.args.map {|arg| type(arg) }
+              args = normalize_args(type_name, type.args).map {|arg| type(arg) }
               Name::Alias.new(name: type_name, args: args)
             when RBS::Types::Union
               Union.build(types: type.types.map {|ty| type(ty) })
@@ -247,7 +270,8 @@ def type_param(type_param)
             name: type_param.name,
             upper_bound: type_opt(type_param.upper_bound_type),
             variance: type_param.variance,
-            unchecked: type_param.unchecked?
+            unchecked: type_param.unchecked?,
+            default_type: type_opt(type_param.default_type)
           )
         end
 
diff --git a/lib/steep/interface/builder.rb b/lib/steep/interface/builder.rb
index 82e6088c7..486c284dc 100644
--- a/lib/steep/interface/builder.rb
+++ b/lib/steep/interface/builder.rb
@@ -50,7 +50,7 @@ def validate_fvs(name, type)
         end
 
         def upper_bound(a)
-          variable_bounds.fetch(a, AST::Builtin::Object.instance_type)
+          variable_bounds.fetch(a, Interface::TypeParam::IMPLICIT_UPPER_BOUND)
         end
       end
 
@@ -494,7 +494,7 @@ def tuple_shape(tuple)
                   block: nil
                 ),
                 MethodType.new(
-                  type_params: [TypeParam.new(name: :T, upper_bound: nil, variance: :invariant, unchecked: false)],
+                  type_params: [TypeParam.new(name: :T, upper_bound: nil, variance: :invariant, unchecked: false, default_type: nil)],
                   type: Function.new(
                     params: Function::Params.build(
                       required: [
@@ -508,7 +508,7 @@ def tuple_shape(tuple)
                   block: nil
                 ),
                 MethodType.new(
-                  type_params: [TypeParam.new(name: :T, upper_bound: nil, variance: :invariant, unchecked: false)],
+                  type_params: [TypeParam.new(name: :T, upper_bound: nil, variance: :invariant, unchecked: false, default_type: nil)],
                   type: Function.new(
                     params: Function::Params.build(required: [AST::Types::Literal.new(value: index)]),
                     return_type: AST::Types::Union.build(types: [elem_type, AST::Types::Var.new(name: :T)]),
@@ -658,7 +658,7 @@ def record_shape(record)
                   block: nil
                 ),
                 MethodType.new(
-                  type_params: [TypeParam.new(name: :T, upper_bound: nil, variance: :invariant, unchecked: false)],
+                  type_params: [TypeParam.new(name: :T, upper_bound: nil, variance: :invariant, unchecked: false, default_type: nil)],
                   type: Function.new(
                     params: Function::Params.build(required: [key_type, AST::Types::Var.new(name: :T)]),
                     return_type: AST::Types::Union.build(types: [value_type, AST::Types::Var.new(name: :T)]),
@@ -667,7 +667,7 @@ def record_shape(record)
                   block: nil
                 ),
                 MethodType.new(
-                  type_params: [TypeParam.new(name: :T, upper_bound: nil, variance: :invariant, unchecked: false)],
+                  type_params: [TypeParam.new(name: :T, upper_bound: nil, variance: :invariant, unchecked: false, default_type: nil)],
                   type: Function.new(
                     params: Function::Params.build(required: [key_type]),
                     return_type: AST::Types::Union.build(types: [value_type, AST::Types::Var.new(name: :T)]),
diff --git a/lib/steep/interface/type_param.rb b/lib/steep/interface/type_param.rb
index e848f01a5..486f2377f 100644
--- a/lib/steep/interface/type_param.rb
+++ b/lib/steep/interface/type_param.rb
@@ -8,13 +8,15 @@ class TypeParam
       attr_reader :variance
       attr_reader :unchecked
       attr_reader :location
+      attr_reader :default_type
 
-      def initialize(name:, upper_bound:, variance:, unchecked:, location: nil)
+      def initialize(name:, upper_bound:, variance:, unchecked:, location: nil, default_type:)
         @name = name
         @upper_bound = upper_bound
         @variance = variance
         @unchecked = unchecked
         @location = location
+        @default_type = default_type
       end
 
       def ==(other)
@@ -22,13 +24,14 @@ def ==(other)
           other.name == name &&
           other.upper_bound == upper_bound &&
           other.variance == variance &&
-          other.unchecked == unchecked
+          other.unchecked == unchecked &&
+          other.default_type == default_type
       end
 
       alias eql? ==
 
       def hash
-        name.hash ^ upper_bound.hash ^ variance.hash ^ unchecked.hash
+        name.hash ^ upper_bound.hash ^ variance.hash ^ unchecked.hash ^ default_type.hash
       end
 
       def self.rename(params, conflicting_names = params.map(&:name), new_names = conflicting_names.map {|n| AST::Types::Var.fresh_name(n) })
@@ -46,7 +49,8 @@ def self.rename(params, conflicting_names = params.map(&:name), new_names = conf
                   upper_bound: param.upper_bound&.subst(subst),
                   variance: param.variance,
                   unchecked: param.unchecked,
-                  location: param.location
+                  location: param.location,
+                  default_type: param.default_type&.subst(subst)
                 )
               else
                 param
@@ -82,19 +86,28 @@ def to_s
         buf
       end
 
-      def update(name: self.name, upper_bound: self.upper_bound, variance: self.variance, unchecked: self.unchecked, location: self.location)
+      def update(name: self.name, upper_bound: self.upper_bound, variance: self.variance, unchecked: self.unchecked, location: self.location, default_type: self.default_type)
         TypeParam.new(
           name: name,
           upper_bound: upper_bound,
           variance: variance,
           unchecked: unchecked,
-          location: location
+          location: location,
+          default_type: default_type
         )
       end
 
       def subst(s)
         if u = upper_bound
-          update(upper_bound: u.subst(s))
+          ub = u.subst(s)
+        end
+
+        if d = default_type
+          dt = d.subst(s)
+        end
+
+        if ub || dt
+          update(upper_bound: ub, default_type: dt)
         else
           self
         end
diff --git a/lib/steep/signature/validator.rb b/lib/steep/signature/validator.rb
index a0a8c19fd..3c47166d4 100644
--- a/lib/steep/signature/validator.rb
+++ b/lib/steep/signature/validator.rb
@@ -101,7 +101,8 @@ def validate_type_application_constraints(type_name, type_params, type_args, loc
                     name: param.name,
                     upper_bound: upper_bound_type,
                     variance: param.variance,
-                    unchecked: param.unchecked?
+                    unchecked: param.unchecked?,
+                    default_type: factory.type_opt(param.default_type)
                   ),
                   location: location
                 )
diff --git a/lib/steep/type_construction.rb b/lib/steep/type_construction.rb
index 725bbde45..b3ee5cadf 100644
--- a/lib/steep/type_construction.rb
+++ b/lib/steep/type_construction.rb
@@ -407,7 +407,8 @@ def for_module(node, new_module_name)
             name: param.name,
             upper_bound: checker.factory.type_opt(param.upper_bound_type),
             variance: param.variance,
-            unchecked: param.unchecked?
+            unchecked: param.unchecked?,
+            default_type: checker.factory.type_opt(param.default_type)
           )
         end
         variable_context = TypeInference::Context::TypeVariableContext.new(type_params)
@@ -497,7 +498,8 @@ def for_class(node, new_class_name, super_class_name)
           upper_bound: type_param.upper_bound_type&.yield_self {|t| checker.factory.type(t) },
           variance: type_param.variance,
           unchecked: type_param.unchecked?,
-          location: type_param.location
+          location: type_param.location,
+          default_type: checker.factory.type_opt(type_param.default_type)
         )
       end
       variable_context = TypeInference::Context::TypeVariableContext.new(type_params)
diff --git a/sig/steep/ast/types/factory.rbs b/sig/steep/ast/types/factory.rbs
index 42f9f6d8b..f085e9407 100644
--- a/sig/steep/ast/types/factory.rbs
+++ b/sig/steep/ast/types/factory.rbs
@@ -20,6 +20,8 @@ module Steep
 
         def type_name_resolver: () -> TypeNameResolver
 
+        def normalize_args: (RBS::TypeName type_name, Array[RBS::Types::t]) -> Array[RBS::Types::t]
+
         def type: (RBS::Types::t `type`) -> t
 
         def type_opt: (RBS::Types::t? `type`) -> t?
diff --git a/sig/steep/interface/type_param.rbs b/sig/steep/interface/type_param.rbs
index 06fa1f666..b08edac40 100644
--- a/sig/steep/interface/type_param.rbs
+++ b/sig/steep/interface/type_param.rbs
@@ -21,7 +21,9 @@ module Steep
 
       attr_reader location: loc?
 
-      def initialize: (name: Symbol, upper_bound: AST::Types::t?, variance: variance, unchecked: bool, ?location: loc?) -> void
+      attr_reader default_type: AST::Types::t?
+
+      def initialize: (name: Symbol, upper_bound: AST::Types::t?, variance: variance, unchecked: bool, ?location: loc?, default_type: AST::Types::t?) -> void
 
       def ==: (untyped other) -> bool
 
@@ -41,7 +43,7 @@ module Steep
 
       def to_s: () -> String
 
-      def update: (?name: Symbol, ?upper_bound: AST::Types::t?, ?variance: variance, ?unchecked: bool, ?location: loc?) -> TypeParam
+      def update: (?name: Symbol, ?upper_bound: AST::Types::t?, ?variance: variance, ?unchecked: bool, ?location: loc?, ?default_type: AST::Types::t?) -> TypeParam
 
       def subst: (Substitution s) -> TypeParam
     end
diff --git a/test/subtyping_test.rb b/test/subtyping_test.rb
index 739be661e..0e5541148 100644
--- a/test/subtyping_test.rb
+++ b/test/subtyping_test.rb
@@ -1009,7 +1009,8 @@ def type_params(checker, **params)
         name: name,
         upper_bound: upper_bound,
         variance: :invariant,
-        unchecked: false
+        unchecked: false,
+        default_type: nil
       )
     end
   end
diff --git a/test/type_check_test.rb b/test/type_check_test.rb
index 795737a9d..676702bc0 100644
--- a/test/type_check_test.rb
+++ b/test/type_check_test.rb
@@ -1433,6 +1433,8 @@ def foo: () -> void
   def test_type_assertion__type_error
     run_type_check_test(
       signatures: {
+        "a.rbs" => <<~RBS
+        RBS
       },
       code: {
         "a.rb" => <<~RUBY
@@ -2216,4 +2218,39 @@ def foo(x)
       YAML
     )
   end
+
+  def test_generics_upperbound_default
+    run_type_check_test(
+      signatures: {
+        "a.rbs" => <<~RBS
+          class Foo[X = Integer]
+            def foo: (X) -> X
+          end
+        RBS
+      },
+      code: {
+        "a.rb" => <<~RUBY
+          class Foo
+            def foo(x)
+              x
+            end
+          end
+
+          x = Foo.new
+          x.foo(1) + 1
+
+          y = Foo.new #: Foo[String]
+          y.foo("foo") + ""
+
+          z = Foo.new #: Foo
+          z.foo(1) + 1
+        RUBY
+      },
+      expectations: <<~YAML
+        ---
+        - file: a.rb
+          diagnostics: []
+      YAML
+    )
+  end
 end