diff --git a/NEWS b/NEWS
index 3346d38ea898..4410bfc0001a 100644
--- a/NEWS
+++ b/NEWS
@@ -7,6 +7,10 @@ PHP                                                                        NEWS
     registrations are freed while still reachable from the cycle collector.
     (Ilia Alshanetsky)
 
+- Session:
+  . Fixed session data loss in the files save handler when writing the session
+    file fails after it was already truncated. (Ilia Alshanetsky)
+
 
 24 Sep 2026, PHP 8.4.26
 
diff --git a/ext/session/mod_files.c b/ext/session/mod_files.c
index 74e77973405b..bf495d815dab 100644
--- a/ext/session/mod_files.c
+++ b/ext/session/mod_files.c
@@ -236,11 +236,6 @@ static zend_result ps_files_write(ps_files *data, zend_string *key, zend_string
 		return FAILURE;
 	}
 
-	/* Truncate file if the amount of new data is smaller than the existing data set. */
-	if (ZSTR_LEN(val) < data->st_size) {
-		php_ignore_value(ftruncate(data->fd, 0));
-	}
-
 #ifdef HAVE_PWRITE
 	n = pwrite(data->fd, ZSTR_VAL(val), ZSTR_LEN(val), 0);
 #else
@@ -274,6 +269,12 @@ static zend_result ps_files_write(ps_files *data, zend_string *key, zend_string
 		return FAILURE;
 	}
 
+	if (ZSTR_LEN(val) < data->st_size) {
+		php_ignore_value(ftruncate(data->fd, ZSTR_LEN(val)));
+	}
+
+	data->st_size = ZSTR_LEN(val);
+
 	return SUCCESS;
 }
 
diff --git a/ext/session/tests/session_files_repeated_write.phpt b/ext/session/tests/session_files_repeated_write.phpt
new file mode 100644
index 000000000000..d6f3446ba27f
--- /dev/null
+++ b/ext/session/tests/session_files_repeated_write.phpt
@@ -0,0 +1,54 @@
+--TEST--
+Session files handler truncates repeated writes to the current data length
+--EXTENSIONS--
+session
+--INI--
+session.save_handler=files
+session.save_path={PWD}
+session.use_cookies=0
+session.cache_limiter=
+session.gc_probability=0
+--FILE--
+<?php
+$id = 'session-files-repeated-write';
+$file = __DIR__ . '/sess_' . $id;
+$handler = new SessionHandler();
+session_set_save_handler($handler);
+$values = ['longer data', 'short', 'SHORT', 'much longer data', ''];
+
+for ($count = 1; $count <= count($values); $count++) {
+    file_put_contents($file, '');
+    session_id($id);
+    session_start();
+    foreach (array_slice($values, 0, $count) as $value) {
+        var_dump($handler->write($id, $value));
+    }
+    session_abort();
+    var_dump(file_get_contents($file));
+}
+?>
+--CLEAN--
+<?php
+unlink(__DIR__ . '/sess_session-files-repeated-write');
+?>
+--EXPECT--
+bool(true)
+string(11) "longer data"
+bool(true)
+bool(true)
+string(5) "short"
+bool(true)
+bool(true)
+bool(true)
+string(5) "SHORT"
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+string(16) "much longer data"
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+string(0) ""
diff --git a/ext/session/tests/session_write_failure_keeps_data.phpt b/ext/session/tests/session_write_failure_keeps_data.phpt
new file mode 100644
index 000000000000..7ef38fccea18
--- /dev/null
+++ b/ext/session/tests/session_write_failure_keeps_data.phpt
@@ -0,0 +1,59 @@
+--TEST--
+Session files handler must not truncate session file when write fails
+--EXTENSIONS--
+session
+posix
+pcntl
+--INI--
+error_reporting=E_ALL
+display_errors=1
+session.use_strict_mode=0
+session.save_handler=files
+session.serialize_handler=php
+--FILE--
+<?php
+$dir = __DIR__ . '/session_write_failure_keeps_data';
+mkdir($dir);
+ini_set('session.save_path', $dir);
+
+$sid = 'write-failure';
+$file = $dir . '/sess_' . $sid;
+
+ob_start();
+
+session_id($sid);
+session_start();
+$_SESSION['data'] = 'original data';
+session_write_close();
+
+$before = file_get_contents($file);
+echo "Original contents: ";
+var_dump($before);
+
+// A zero file size limit makes the next write fail before any byte lands;
+// SIGXFSZ has to be ignored or the process is killed instead.
+pcntl_signal(SIGXFSZ, SIG_IGN);
+echo "Set file size limit to zero: ";
+var_dump(posix_setrlimit(POSIX_RLIMIT_FSIZE, 0, POSIX_RLIMIT_INFINITY));
+
+session_start();
+$_SESSION['data'] = 'new';
+@session_write_close();
+
+echo "Contents unchanged after failed write: ";
+var_dump(file_get_contents($file) === $before);
+
+ob_end_flush();
+?>
+--CLEAN--
+<?php
+$dir = __DIR__ . '/session_write_failure_keeps_data';
+foreach (glob($dir . '/sess_*') as $f) {
+    unlink($f);
+}
+rmdir($dir);
+?>
+--EXPECT--
+Original contents: string(26) "data|s:13:"original data";"
+Set file size limit to zero: bool(true)
+Contents unchanged after failed write: bool(true)