diff --git a/lib/minitest/spec.rb b/lib/minitest/spec.rb
index 07eeb3a0..738b74e0 100644
--- a/lib/minitest/spec.rb
+++ b/lib/minitest/spec.rb
@@ -8,6 +8,9 @@ def infect_an_assertion meth, new_name, dont_flip = false # :nodoc:
     # warn "%-22p -> %p %p" % [meth, new_name, dont_flip]
     self.class_eval <<-EOM, __FILE__, __LINE__ + 1
       def #{new_name} *args
+        where = Minitest.filter_backtrace(caller).first
+        where = where.split(/:in /, 2).first # clean up noise
+        warn "DEPRECATED: global use of #{new_name} from #\{where}. Use _(obj).#{new_name} instead. This will fail in Minitest 6."
         Minitest::Expectation.new(self, Minitest::Spec.current).#{new_name}(*args)
       end
     EOM
diff --git a/test/minitest/test_minitest_spec.rb b/test/minitest/test_minitest_spec.rb
index 3ce31e30..2e108d9c 100644
--- a/test/minitest/test_minitest_spec.rb
+++ b/test/minitest/test_minitest_spec.rb
@@ -535,6 +535,18 @@ def assert_triggered expected = "blah", klass = Minitest::Assertion
 
       assert_equal "Calling #must_equal outside of test.", e.message
     end
+
+    it "deprecates expectation used without _" do
+      skip "N/A" if ENV["MT_NO_EXPECTATIONS"]
+
+      @assertion_count += 3
+
+      exp = /DEPRECATED: global use of must_equal from/
+
+      assert_output "", exp do
+        (1 + 1).must_equal 2
+      end
+    end
   end
 
   it "needs to verify throw" do