diff --git a/src/uu/mktemp/src/mktemp.rs b/src/uu/mktemp/src/mktemp.rs
index c285e9c90..5e3b8aa58 100644
--- a/src/uu/mktemp/src/mktemp.rs
+++ b/src/uu/mktemp/src/mktemp.rs
@@ -193,9 +193,22 @@ struct Params {
 /// assert_eq!(find_last_contiguous_block_of_xs("aXbXcX"), None);
 /// ```
 fn find_last_contiguous_block_of_xs(s: &str) -> Option<(usize, usize)> {
-    let j = s.rfind("XXX")? + 3;
-    let i = s[..j].rfind(|c| c != 'X').map_or(0, |i| i + 1);
-    Some((i, j))
+    let bytes = s.as_bytes();
+
+    // Find the index of the last 'X'.
+    let end = bytes.iter().rposition(|&b| b == b'X')?;
+
+    // Walk left to find the start of the run of Xs that ends at `end`.
+    let mut start = end;
+    while start > 0 && bytes[start - 1] == b'X' {
+        start -= 1;
+    }
+
+    if end + 1 - start >= 3 {
+        Some((start, end + 1))
+    } else {
+        None
+    }
 }
 
 impl Params {
diff --git a/tests/by-util/test_mktemp.rs b/tests/by-util/test_mktemp.rs
index 405c7bfee..cedcb4927 100644
--- a/tests/by-util/test_mktemp.rs
+++ b/tests/by-util/test_mktemp.rs
@@ -30,6 +30,7 @@ static TEST_TEMPLATE7: &str = "XXXtemplate";
 static TEST_TEMPLATE8: &str = "tempXXXl/ate";
 #[cfg(windows)]
 static TEST_TEMPLATE8: &str = "tempXXXl\\ate";
+static TEST_TEMPLATE9: &str = "XXX_XX";
 
 #[cfg(not(windows))]
 const TMPDIR: &str = "TMPDIR";
@@ -109,6 +110,11 @@ fn test_mktemp_mktemp() {
         .env(TMPDIR, &pathname)
         .arg(TEST_TEMPLATE8)
         .fails();
+    scene
+        .ucmd()
+        .env(TMPDIR, &pathname)
+        .arg(TEST_TEMPLATE9)
+        .fails();
 }
 
 #[test]
@@ -168,6 +174,12 @@ fn test_mktemp_mktemp_t() {
         .no_stdout()
         .stderr_contains("invalid suffix")
         .stderr_contains("contains directory separator");
+    scene
+        .ucmd()
+        .env(TMPDIR, &pathname)
+        .arg("-t")
+        .arg(TEST_TEMPLATE9)
+        .fails();
 }
 
 #[test]
@@ -224,6 +236,12 @@ fn test_mktemp_make_temp_dir() {
         .arg("-d")
         .arg(TEST_TEMPLATE8)
         .fails();
+    scene
+        .ucmd()
+        .env(TMPDIR, &pathname)
+        .arg("-d")
+        .arg(TEST_TEMPLATE9)
+        .fails();
 }
 
 #[test]
@@ -280,6 +298,12 @@ fn test_mktemp_dry_run() {
         .arg("-u")
         .arg(TEST_TEMPLATE8)
         .fails();
+    scene
+        .ucmd()
+        .env(TMPDIR, &pathname)
+        .arg("-u")
+        .arg(TEST_TEMPLATE9)
+        .fails();
 }
 
 #[test]
@@ -367,6 +391,13 @@ fn test_mktemp_suffix() {
         .arg("suf")
         .arg(TEST_TEMPLATE8)
         .fails();
+    scene
+        .ucmd()
+        .env(TMPDIR, &pathname)
+        .arg("--suffix")
+        .arg("suf")
+        .arg(TEST_TEMPLATE9)
+        .fails();
 }
 
 #[test]
@@ -424,6 +455,12 @@ fn test_mktemp_tmpdir() {
         .arg(pathname)
         .arg(TEST_TEMPLATE8)
         .fails();
+    scene
+        .ucmd()
+        .arg("-p")
+        .arg(pathname)
+        .arg(TEST_TEMPLATE9)
+        .fails();
 }
 
 #[test]