diff --git a/config/default.yml b/config/default.yml
index 63fdd04..22bd7c3 100644
--- a/config/default.yml
+++ b/config/default.yml
@@ -2,3 +2,8 @@ TypeToolkit/DontExpectUnexpectedNil:
   Description: "Detects misuse of UnexpectedNilError (rescuing, raising, or asserting it)."
   Enabled: true
   VersionAdded: "0.1.0"
+
+TypeToolkit/PreferNotNil:
+  Description: "Replaces T.must assertions with Type Toolkit's not_nil! assertion."
+  Enabled: true
+  VersionAdded: "0.1.0"
diff --git a/lib/rubocop-type_toolkit.rb b/lib/rubocop-type_toolkit.rb
index d55e476..e716759 100644
--- a/lib/rubocop-type_toolkit.rb
+++ b/lib/rubocop-type_toolkit.rb
@@ -3,3 +3,4 @@
 require "rubocop"
 require_relative "rubocop/cop/type_toolkit/plugin"
 require_relative "rubocop/cop/type_toolkit/dont_expect_unexpected_nil"
+require_relative "rubocop/cop/type_toolkit/prefer_not_nil"
diff --git a/lib/rubocop/cop/type_toolkit/plugin.rb b/lib/rubocop/cop/type_toolkit/plugin.rb
index a46c367..7bdcd89 100644
--- a/lib/rubocop/cop/type_toolkit/plugin.rb
+++ b/lib/rubocop/cop/type_toolkit/plugin.rb
@@ -13,7 +13,7 @@ def about
             name: "rubocop-type_toolkit",
             version: ::TypeToolkit::VERSION,
             homepage: "https://github.com/Shopify/type_toolkit",
-            description: "Detects misuse of UnexpectedNilError.",
+            description: "RuboCop rules for Type Toolkit.",
           )
         end
 
diff --git a/lib/rubocop/cop/type_toolkit/prefer_not_nil.rb b/lib/rubocop/cop/type_toolkit/prefer_not_nil.rb
new file mode 100644
index 0000000..ff3968b
--- /dev/null
+++ b/lib/rubocop/cop/type_toolkit/prefer_not_nil.rb
@@ -0,0 +1,96 @@
+# typed: true
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module TypeToolkit
+      # Replaces Sorbet's `T.must(value)` assertion with Type Toolkit's `value.not_nil!` assertion.
+      class PreferNotNil < Base
+        extend AutoCorrector
+
+        MSG = "Use `.not_nil!` instead of `T.must()`."
+        RESTRICT_ON_SEND = [:must].freeze
+
+        COMMA_BYTE = ",".ord
+        private_constant :COMMA_BYTE
+
+        KEYWORD_EXPRESSION_TYPES = [:defined?, :super, :yield, :zsuper].freeze
+        private_constant :KEYWORD_EXPRESSION_TYPES
+
+        #: (RuboCop::AST::SendNode) -> void
+        def on_send(node)
+          return unless (argument = extract_t_must_argument(node))
+
+          if nested_t_must?(node)
+            add_offense(node, message: MSG)
+          else
+            replacement = replacement_for(argument)
+            correction = correction_for(node, argument, replacement)
+
+            add_offense(node, message: MSG) do |corrector|
+              corrector.replace(node, correction)
+            end
+          end
+        end
+
+        private
+
+        #: (RuboCop::AST::SendNode) -> RuboCop::AST::Node?
+        def extract_t_must_argument(node)
+          receiver = node.receiver
+          return unless receiver.is_a?(RuboCop::AST::ConstNode)
+          return unless receiver.short_name == :T && node.method?(:must) && node.arguments.one?
+
+          namespace = receiver.namespace
+          return unless namespace.nil? || namespace.cbase_type?
+
+          argument = node.first_argument
+          return unless argument
+          return if argument.splat_type? || argument.kwsplat_type?
+
+          argument
+        end
+
+        #: (RuboCop::AST::Node) -> String
+        def replacement_for(argument)
+          source = argument.source
+          source = "(#{source})" if requires_parentheses?(argument)
+          "#{source}.not_nil!"
+        end
+
+        #: (RuboCop::AST::SendNode, RuboCop::AST::Node, String) -> String
+        def correction_for(node, argument, replacement)
+          return replacement unless node.multiline?
+
+          grouped_range = node.source_range.with(begin_pos: node.loc.begin.begin_pos, end_pos: node.loc.end.end_pos)
+          grouped_source = grouped_range.source
+          comma_offset = argument.source_range.end_pos - grouped_range.begin_pos
+          grouped_source.slice!(comma_offset) if grouped_source.getbyte(comma_offset) == COMMA_BYTE
+          "#{grouped_source}.not_nil!"
+        end
+
+        #: (RuboCop::AST::SendNode) -> bool
+        def nested_t_must?(node)
+          node.each_ancestor(:send).any? do |ancestor|
+            ancestor.is_a?(RuboCop::AST::SendNode) && extract_t_must_argument(ancestor)
+          end
+        end
+
+        #: (RuboCop::AST::Node) -> bool
+        def requires_parentheses?(argument)
+          return false if argument.begin_type?
+
+          if argument.is_a?(RuboCop::AST::SendNode)
+            return true if argument.operator_method?
+            return true if argument.arguments? && !argument.parenthesized_call?
+          end
+          return true if argument.range_type? || argument.operator_keyword?
+          return true if argument.if_type? || argument.assignment?
+          return true if argument.any_block_type?
+
+          KEYWORD_EXPRESSION_TYPES.include?(argument.type)
+        end
+      end
+    end
+  end
+end
diff --git a/spec/rubocop/cop/type_toolkit/prefer_not_nil_spec.rb b/spec/rubocop/cop/type_toolkit/prefer_not_nil_spec.rb
new file mode 100644
index 0000000..eb1cfce
--- /dev/null
+++ b/spec/rubocop/cop/type_toolkit/prefer_not_nil_spec.rb
@@ -0,0 +1,229 @@
+# typed: true
+# frozen_string_literal: true
+
+require "spec_helper"
+require "rubocop"
+require "rubocop/minitest/assert_offense"
+require "rubocop-type_toolkit"
+
+module RuboCop
+  module Cop
+    module TypeToolkit
+      class PreferNotNilSpec < ::Minitest::Spec
+        include RuboCop::Minitest::AssertOffense
+
+        MSG = "TypeToolkit/PreferNotNil: Use `.not_nil!` instead of `T.must()`."
+
+        before do
+          @cop = PreferNotNil.new
+        end
+
+        it "autocorrects T.must" do
+          assert_offense(<<~RUBY)
+            value = T.must(foo)
+                    ^^^^^^^^^^^ #{MSG}
+          RUBY
+
+          assert_correction(<<~RUBY)
+            value = foo.not_nil!
+          RUBY
+        end
+
+        it "autocorrects ::T.must" do
+          assert_offense(<<~RUBY)
+            value = ::T.must(foo)
+                    ^^^^^^^^^^^^^ #{MSG}
+          RUBY
+
+          assert_correction(<<~RUBY)
+            value = foo.not_nil!
+          RUBY
+        end
+
+        it "autocorrects inside string interpolation" do
+          assert_offense(<<~RUBY)
+            string = "\#{T.must(foo)}"
+                        ^^^^^^^^^^^ #{MSG}
+          RUBY
+
+          assert_correction(<<~RUBY)
+            string = "\#{foo.not_nil!}"
+          RUBY
+        end
+
+        it "preserves the precedence of conditional and range expressions" do
+          assert_offense(<<~RUBY)
+            conditional = T.must(condition ? foo : bar)
+                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ #{MSG}
+            range = T.must(foo..bar)
+                    ^^^^^^^^^^^^^^^^ #{MSG}
+          RUBY
+
+          assert_correction(<<~RUBY)
+            conditional = (condition ? foo : bar).not_nil!
+            range = (foo..bar).not_nil!
+          RUBY
+        end
+
+        it "parenthesizes every expression that requires it" do
+          assert_offense(<<~RUBY)
+            operator = T.must(foo + bar)
+                       ^^^^^^^^^^^^^^^^^ #{MSG}
+            logical = T.must(foo || bar)
+                      ^^^^^^^^^^^^^^^^^^ #{MSG}
+            grouped = T.must((foo || bar))
+                      ^^^^^^^^^^^^^^^^^^^^ #{MSG}
+            assignment = T.must(foo = bar)
+                         ^^^^^^^^^^^^^^^^^ #{MSG}
+            block_value = T.must(foo { bar })
+                          ^^^^^^^^^^^^^^^^^^^ #{MSG}
+            defined_value = T.must(defined?(foo))
+                            ^^^^^^^^^^^^^^^^^^^^^ #{MSG}
+            def example
+              T.must(yield foo)
+              ^^^^^^^^^^^^^^^^^ #{MSG}
+            end
+            def implicit_super
+              T.must(super)
+              ^^^^^^^^^^^^^ #{MSG}
+            end
+            def explicit_super
+              T.must(super(foo))
+              ^^^^^^^^^^^^^^^^^^ #{MSG}
+            end
+          RUBY
+
+          assert_correction(<<~RUBY)
+            operator = (foo + bar).not_nil!
+            logical = (foo || bar).not_nil!
+            grouped = (foo || bar).not_nil!
+            assignment = (foo = bar).not_nil!
+            block_value = (foo { bar }).not_nil!
+            defined_value = (defined?(foo)).not_nil!
+            def example
+              (yield foo).not_nil!
+            end
+            def implicit_super
+              (super).not_nil!
+            end
+            def explicit_super
+              (super(foo)).not_nil!
+            end
+          RUBY
+        end
+
+        it "preserves the precedence of command calls" do
+          assert_offense(<<~RUBY)
+            value = T.must(fetch value)
+                    ^^^^^^^^^^^^^^^^^^^ #{MSG}
+          RUBY
+
+          assert_correction(<<~RUBY)
+            value = (fetch value).not_nil!
+          RUBY
+        end
+
+        it "does not add unnecessary parentheses to parenthesized calls" do
+          assert_offense(<<~RUBY)
+            value = T.must(fetch(value))
+                    ^^^^^^^^^^^^^^^^^^^^ #{MSG}
+          RUBY
+
+          assert_correction(<<~RUBY)
+            value = fetch(value).not_nil!
+          RUBY
+        end
+
+        it "preserves comments in multiline calls" do
+          assert_offense(<<~RUBY)
+            value = T.must(
+                    ^^^^^^^ #{MSG}
+              # Proven non-nil by validation.
+              foo,
+            )
+          RUBY
+
+          assert_correction(<<~RUBY)
+            value = (
+              # Proven non-nil by validation.
+              foo
+            ).not_nil!
+          RUBY
+        end
+
+        it "autocorrects multiline calls with whitespace before the method" do
+          assert_offense(<<~RUBY)
+            first = T .must(
+                    ^^^^^^^^ #{MSG}
+              foo,
+            )
+            second = T
+                     ^ #{MSG}
+              .must(
+                bar,
+              )
+          RUBY
+
+          assert_correction(<<~RUBY)
+            first = (
+              foo
+            ).not_nil!
+            second = (
+                bar
+              ).not_nil!
+          RUBY
+        end
+
+        it "removes a trailing comma before an inline comment" do
+          assert_offense(<<~RUBY)
+            value = T.must(
+                    ^^^^^^^ #{MSG}
+              foo, # Proven non-nil.
+            )
+          RUBY
+
+          assert_correction(<<~RUBY)
+            value = (
+              foo # Proven non-nil.
+            ).not_nil!
+          RUBY
+        end
+
+        it "autocorrects nested T.must calls" do
+          assert_offense(<<~RUBY)
+            value = T.must(T.must(foo).bar)
+                    ^^^^^^^^^^^^^^^^^^^^^^^ #{MSG}
+                           ^^^^^^^^^^^ #{MSG}
+          RUBY
+
+          assert_correction(<<~RUBY)
+            value = foo.not_nil!.bar.not_nil!
+          RUBY
+        end
+
+        it "autocorrects three nested T.must calls" do
+          assert_offense(<<~RUBY)
+            value = T.must(T.must(T.must(foo).bar).baz)
+                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ #{MSG}
+                           ^^^^^^^^^^^^^^^^^^^^^^^ #{MSG}
+                                  ^^^^^^^^^^^ #{MSG}
+          RUBY
+
+          assert_correction(<<~RUBY)
+            value = foo.not_nil!.bar.not_nil!.baz.not_nil!
+          RUBY
+        end
+
+        it "ignores other receivers, methods, and argument counts" do
+          assert_no_offenses(<<~RUBY)
+            Other::T.must(foo)
+            object.must(foo)
+            T.let(foo, String)
+            T.must(foo, bar)
+            T.must(*values)
+          RUBY
+        end
+      end
+    end
+  end
+end