diff --git a/config/tcl.m4 b/config/tcl.m4
index 9de31a57156..d226ef99d05 100644
--- a/config/tcl.m4
+++ b/config/tcl.m4
@@ -4,7 +4,7 @@
 
 
 AC_DEFUN([PGAC_PATH_TCLSH],
-[PGAC_PATH_PROGS(TCLSH, [tclsh tcl tclsh8.6 tclsh86 tclsh8.5 tclsh85 tclsh8.4 tclsh84])
+[PGAC_PATH_PROGS(TCLSH, [tclsh tcl tclsh8.6 tclsh86])
 AC_ARG_VAR(TCLSH, [Tcl interpreter program (tclsh)])dnl
 if test x"$TCLSH" = x""; then
   AC_MSG_ERROR([Tcl shell not found])
diff --git a/configure b/configure
index 5fc85a24519..83609ac988c 100755
--- a/configure
+++ b/configure
@@ -19003,7 +19003,7 @@ fi
 # Check for Tcl configuration script tclConfig.sh
 if test "$with_tcl" = yes; then
     if test -z "$TCLSH"; then
-  for ac_prog in tclsh tcl tclsh8.6 tclsh86 tclsh8.5 tclsh85 tclsh8.4 tclsh84
+  for ac_prog in tclsh tcl tclsh8.6 tclsh86
 do
   # Extract the first word of "$ac_prog", so it can be a program name with args.
 set dummy $ac_prog; ac_word=$2
diff --git a/doc/src/sgml/installation.sgml b/doc/src/sgml/installation.sgml
index 37a0473d03e..8b98938d2a1 100644
--- a/doc/src/sgml/installation.sgml
+++ b/doc/src/sgml/installation.sgml
@@ -265,7 +265,7 @@
       To build the <application>PL/Tcl</application>
       procedural language, you of course need a <productname>Tcl</productname>
       installation.  The minimum required version is
-      <productname>Tcl</productname> 8.4.
+      <productname>Tcl</productname> 8.6.
      </para>
     </listitem>
 
@@ -1936,8 +1936,7 @@ build-postgresql:
          Tcl interpreter program.  This will be used to
          determine the dependencies for building PL/Tcl.
          If this is not set, the following are probed in this
-         order: <literal>tclsh tcl tclsh8.6 tclsh86 tclsh8.5 tclsh85
-         tclsh8.4 tclsh84</literal>.
+         order: <literal>tclsh tcl tclsh8.6 tclsh86</literal>.
         </para>
        </listitem>
       </varlistentry>
diff --git a/src/pl/tcl/pltcl.c b/src/pl/tcl/pltcl.c
index 85e83bbf1e3..c8a9363aaa1 100644
--- a/src/pl/tcl/pltcl.c
+++ b/src/pl/tcl/pltcl.c
@@ -50,14 +50,9 @@ PG_MODULE_MAGIC_EXT(
 	((TCL_MAJOR_VERSION > maj) || \
 	 (TCL_MAJOR_VERSION == maj && TCL_MINOR_VERSION >= min))
 
-/* Insist on Tcl >= 8.4 */
-#if !HAVE_TCL_VERSION(8,4)
-#error PostgreSQL only supports Tcl 8.4 or later.
-#endif
-
-/* Hack to deal with Tcl 8.6 const-ification without losing compatibility */
-#ifndef CONST86
-#define CONST86
+/* Insist on Tcl >= 8.6 */
+#if !HAVE_TCL_VERSION(8,6)
+#error PostgreSQL only supports Tcl 8.6 or later.
 #endif
 
 #if !HAVE_TCL_VERSION(8,7)
@@ -366,7 +361,7 @@ pltcl_FinalizeNotifier(ClientData clientData)
 }
 
 static void
-pltcl_SetTimer(CONST86 Tcl_Time *timePtr)
+pltcl_SetTimer(const Tcl_Time *timePtr)
 {
 }
 
@@ -392,7 +387,7 @@ pltcl_ServiceModeHook(int mode)
 }
 
 static int
-pltcl_WaitForEvent(CONST86 Tcl_Time *timePtr)
+pltcl_WaitForEvent(const Tcl_Time *timePtr)
 {
 	return 0;
 }