diff --git a/lib/elixir/lib/protocol.ex b/lib/elixir/lib/protocol.ex
index 1e4c3911ad6..df6d9df1632 100644
--- a/lib/elixir/lib/protocol.ex
+++ b/lib/elixir/lib/protocol.ex
@@ -614,10 +614,12 @@ defmodule Protocol do
         {impl_for!, definitions} = List.keytake(definitions, {:impl_for!, 1}, 0)
         {struct_impl_for, definitions} = List.keytake(definitions, {:struct_impl_for, 1}, 0)
 
+        protocol_funs = get_protocol_functions(protocol_def)
+
         protocol_def = change_protocol(protocol_def, types)
         impl_for = change_impl_for(impl_for, protocol, types)
         struct_impl_for = change_struct_impl_for(struct_impl_for, protocol, types, structs)
-        new_signatures = new_signatures(definitions, protocol, types)
+        new_signatures = new_signatures(definitions, protocol_funs, protocol, types)
 
         definitions = [protocol_def, impl_for, impl_for!, struct_impl_for] ++ definitions
         signatures = Enum.into(new_signatures, signatures)
@@ -628,7 +630,7 @@ defmodule Protocol do
     end
   end
 
-  defp new_signatures(definitions, protocol, types) do
+  defp new_signatures(definitions, protocol_funs, protocol, types) do
     alias Module.Types.Descr
 
     clauses =
@@ -666,9 +668,10 @@ defmodule Protocol do
       end
 
     new_signatures =
-      for {{fun, arity}, :def, _, _} <- definitions do
+      for {{_fun, arity} = fun_arity, :def, _, _} <- definitions,
+          fun_arity in protocol_funs do
         rest = List.duplicate(Descr.term(), arity - 1)
-        {{fun, arity}, {:strong, nil, [{[domain | rest], Descr.dynamic()}]}}
+        {fun_arity, {:strong, nil, [{[domain | rest], Descr.dynamic()}]}}
       end
 
     [
@@ -677,6 +680,13 @@ defmodule Protocol do
     ] ++ new_signatures
   end
 
+  defp get_protocol_functions({_name, _kind, _meta, clauses}) do
+    Enum.find_value(clauses, fn
+      {_meta, [:functions], [], clauses} -> clauses
+      _ -> nil
+    end) || raise "could not find protocol functions"
+  end
+
   defp change_protocol({_name, _kind, meta, clauses}, types) do
     clauses =
       Enum.map(clauses, fn
diff --git a/lib/elixir/test/elixir/fixtures/consolidation/sample.ex b/lib/elixir/test/elixir/fixtures/consolidation/sample.ex
index ee7edea3fbc..b1530e20a52 100644
--- a/lib/elixir/test/elixir/fixtures/consolidation/sample.ex
+++ b/lib/elixir/test/elixir/fixtures/consolidation/sample.ex
@@ -4,4 +4,9 @@ defprotocol Protocol.ConsolidationTest.Sample do
   @deprecated "Reason"
   @spec ok(t) :: boolean
   def ok(term)
+
+  # Not a protocol function. While this is not "officially" supported,
+  # it does happen in practice, so we need to make sure we preserve
+  # its signature.
+  Kernel.def(regular_fun(term), do: term + 1)
 end
diff --git a/lib/elixir/test/elixir/protocol/consolidation_test.exs b/lib/elixir/test/elixir/protocol/consolidation_test.exs
index 64f35f88e6a..9a209961521 100644
--- a/lib/elixir/test/elixir/protocol/consolidation_test.exs
+++ b/lib/elixir/test/elixir/protocol/consolidation_test.exs
@@ -234,6 +234,12 @@ defmodule Protocol.ConsolidationTest do
       assert %{{:ok, 1} => %{sig: {:strong, nil, clauses}}} = exports
       assert clauses == [{[none()], dynamic()}]
     end
+
+    test "handles regular function definitions" do
+      exports = exports(sample_binary())
+
+      assert %{{:regular_fun, 1} => %{sig: :none}} = exports
+    end
   end
 
   test "consolidation errors on missing BEAM files" do