diff --git a/src/nvim/ex_cmds.lua b/src/nvim/ex_cmds.lua
index 845ea4bb15..b0938fa711 100644
--- a/src/nvim/ex_cmds.lua
+++ b/src/nvim/ex_cmds.lua
@@ -714,15 +714,9 @@ module.cmds = {
     addr_type='ADDR_OTHER',
     func='ex_debuggreedy',
   },
-  {
-    command='def',
-    flags=bit.bor(EXTRA, BANG, SBOXOK, CMDWIN, LOCK_OK),
-    addr_type='ADDR_NONE',
-    func='ex_ni',
-  },
   {
     command='defer',
-    flags=bit.bor(NEEDARG, EXTRA, NOTRLCOM, CMDWIN, LOCK_OK),
+    flags=bit.bor(NEEDARG, EXTRA, NOTRLCOM, SBOXOK, CMDWIN, LOCK_OK),
     addr_type='ADDR_NONE',
     func='ex_call',
   },
diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index 246aa0aace..b65fa086f4 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -2992,6 +2992,11 @@ char *find_ex_command(exarg_T *eap, int *full)
     }
     assert(eap->cmdidx >= 0);
 
+    if (len == 3 && strncmp("def", eap->cmd, 3) == 0) {
+      // Make :def an unknown command to avoid confusing behavior. #23149
+      eap->cmdidx = CMD_SIZE;
+    }
+
     for (; (int)eap->cmdidx < CMD_SIZE;
          eap->cmdidx = (cmdidx_T)((int)eap->cmdidx + 1)) {
       if (strncmp(cmdnames[(int)eap->cmdidx].cmd_name, eap->cmd,
@@ -3146,6 +3151,11 @@ void f_fullcommand(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 
 cmdidx_T excmd_get_cmdidx(const char *cmd, size_t len)
 {
+  if (len == 3 && strncmp("def", cmd, 3) == 0) {
+    // Make :def an unknown command to avoid confusing behavior. #23149
+    return CMD_SIZE;
+  }
+
   cmdidx_T idx;
 
   if (!one_letter_cmd(cmd, &idx)) {
diff --git a/test/functional/ex_cmds/excmd_spec.lua b/test/functional/ex_cmds/excmd_spec.lua
index e243f3c524..b313d6428c 100644
--- a/test/functional/ex_cmds/excmd_spec.lua
+++ b/test/functional/ex_cmds/excmd_spec.lua
@@ -2,6 +2,7 @@ local helpers = require("test.functional.helpers")(after_each)
 local command = helpers.command
 local eq = helpers.eq
 local clear = helpers.clear
+local funcs = helpers.funcs
 local pcall_err = helpers.pcall_err
 local assert_alive = helpers.assert_alive
 
@@ -26,5 +27,18 @@ describe('Ex cmds', function()
       pcall_err(command, ':bdelete 9999999999999999999999999999999999999999'))
     assert_alive()
   end)
-end)
 
+  it(':def is an unknown command #23149', function()
+    eq('Vim:E492: Not an editor command: def', pcall_err(command, 'def'))
+    eq(1, funcs.exists(':d'))
+    eq('delete', funcs.fullcommand('d'))
+    eq(1, funcs.exists(':de'))
+    eq('delete', funcs.fullcommand('de'))
+    eq(0, funcs.exists(':def'))
+    eq('', funcs.fullcommand('def'))
+    eq(1, funcs.exists(':defe'))
+    eq('defer', funcs.fullcommand('defe'))
+    eq(2, funcs.exists(':defer'))
+    eq('defer', funcs.fullcommand('defer'))
+  end)
+end)