diff --git a/runtest b/runtest
index 88617c1a6..09b9d491b 100755
--- a/runtest
+++ b/runtest
@@ -1,5 +1,5 @@
 #!/bin/sh
-TCL_VERSIONS="8.5 8.6 8.7"
+TCL_VERSIONS="8.5 8.6 8.7 9.0"
 TCLSH=""
 
 for VERSION in $TCL_VERSIONS; do
diff --git a/tests/instances.tcl b/tests/instances.tcl
index d93b36b33..8f0a23bd2 100644
--- a/tests/instances.tcl
+++ b/tests/instances.tcl
@@ -10,9 +10,9 @@
 # (RSALv2); or (b) the Server Side Public License v1 (SSPLv1); or (c) the
 # GNU Affero General Public License v3 (AGPLv3).
 
-package require Tcl 8.5
+package require Tcl 8.5-10
 
-set tcl_precision 17
+if {$tcl_version < 9.0} { set tcl_precision 17 }
 source ../support/redis.tcl
 source ../support/util.tcl
 source ../support/aofmanifest.tcl
diff --git a/tests/support/cluster.tcl b/tests/support/cluster.tcl
index 3a666844b..cee4a6f1f 100644
--- a/tests/support/cluster.tcl
+++ b/tests/support/cluster.tcl
@@ -14,7 +14,7 @@
 # $c get foo
 # $c close
 
-package require Tcl 8.5
+package require Tcl 8.5-10
 package provide redis_cluster 0.1
 
 namespace eval redis_cluster {}
diff --git a/tests/support/redis.tcl b/tests/support/redis.tcl
index 6c8585587..e497b25e9 100644
--- a/tests/support/redis.tcl
+++ b/tests/support/redis.tcl
@@ -30,7 +30,7 @@
 #
 # vwait forever
 
-package require Tcl 8.5
+package require Tcl 8.5-10
 package provide redis 0.1
 
 source [file join [file dirname [info script]] "response_transformers.tcl"]
@@ -165,6 +165,11 @@ proc ::redis::__dispatch__raw__ {id method argv} {
         set cmd "*[expr {[llength $argv]+1}]\r\n"
         append cmd "$[string length $method]\r\n$method\r\n"
         foreach a $argv {
+            # In Tcl 9.0, only convert to UTF-8 if the string contains non-byte characters
+            # to preserve binary data while handling unicode correctly
+            if {$::tcl_version >= 9.0 && [string match "*\[^\u0000-\u00ff\]*" $a]} {
+                set a [encoding convertto utf-8 $a]
+            }
             append cmd "$[string length $a]\r\n$a\r\n"
         }
         ::redis::redis_write $fd $cmd
diff --git a/tests/support/response_transformers.tcl b/tests/support/response_transformers.tcl
index 99c1ebb65..8b81a38f1 100644
--- a/tests/support/response_transformers.tcl
+++ b/tests/support/response_transformers.tcl
@@ -19,7 +19,7 @@
 # changes in many files) we decided to transform the response to RESP2
 # when running with --force-resp3
 
-package require Tcl 8.5
+package require Tcl 8.5-10
 
 namespace eval response_transformers {}
 
diff --git a/tests/test_helper.tcl b/tests/test_helper.tcl
index 88cb6a76a..b93727baf 100644
--- a/tests/test_helper.tcl
+++ b/tests/test_helper.tcl
@@ -13,9 +13,9 @@
 # Portions of this file are available under BSD3 terms; see REDISCONTRIBUTIONS for more information.
 #
 
-package require Tcl 8.5
+package require Tcl 8.5-10
 
-set tcl_precision 17
+if {$tcl_version < 9.0} { set tcl_precision 17 }
 source tests/support/redis.tcl
 source tests/support/aofmanifest.tcl
 source tests/support/server.tcl
@@ -333,7 +333,7 @@ proc test_server_cron {} {
 }
 
 proc accept_test_clients {fd addr port} {
-    fconfigure $fd -encoding binary
+    fconfigure $fd -translation binary
     fileevent $fd readable [list read_from_test_client $fd]
 }
 
@@ -524,7 +524,7 @@ proc the_end {} {
 # to read the command, execute, reply... all this in a loop.
 proc test_client_main server_port {
     set ::test_server_fd [socket localhost $server_port]
-    fconfigure $::test_server_fd -encoding binary
+    fconfigure $::test_server_fd -translation binary
     send_data_packet $::test_server_fd ready [pid]
     while 1 {
         set bytes [gets $::test_server_fd]
diff --git a/tests/unit/other.tcl b/tests/unit/other.tcl
index b8d5d0fc8..7ab9ab89b 100644
--- a/tests/unit/other.tcl
+++ b/tests/unit/other.tcl
@@ -227,7 +227,7 @@ start_server {tags {"other"}} {
             } else {
                 set fd2 [socket [srv host] [srv port]]
             }
-            fconfigure $fd2 -encoding binary -translation binary
+            fconfigure $fd2 -translation binary
             if {!$::singledb} {
                 puts -nonewline $fd2 "SELECT 9\r\n"
                 flush $fd2