diff --git a/src/uu/uniq/src/uniq.rs b/src/uu/uniq/src/uniq.rs
index 3288241e4ef..6f324929cc3 100644
--- a/src/uu/uniq/src/uniq.rs
+++ b/src/uu/uniq/src/uniq.rs
@@ -2,7 +2,7 @@
 //
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
-// spell-checker:ignore badoption
+// spell-checker:ignore badoption CTYPE
 use clap::{
     Arg, ArgAction, ArgMatches, Command, builder::ValueParser, error::ContextKind, error::Error,
     error::ErrorKind,
@@ -186,6 +186,17 @@ impl Uniq {
         }
     }
 
+    fn is_c_locale() -> bool {
+        for key in ["LC_ALL", "LC_CTYPE", "LANG"] {
+            if let Some(v) = std::env::var_os(key) {
+                if !v.is_empty() {
+                    return v == "C" || v == "POSIX";
+                }
+            }
+        }
+        true
+    }
+
     fn key_end_index(&self, line: &[u8], key_start: usize) -> usize {
         let remainder = &line[key_start..];
         match self.slice_stop {
@@ -194,10 +205,15 @@ impl Uniq {
                 if remainder.is_empty() {
                     return key_start;
                 }
-                if let Ok(valid) = std::str::from_utf8(remainder) {
+                if Self::is_c_locale() {
+                    // for C or POSIX we count bytes
+                    key_start + remainder.len().min(limit)
+                } else if let Ok(valid) = std::str::from_utf8(remainder) {
+                    // for UTF-8 we count characters
                     let prefix_len = Self::char_prefix_len(valid, limit);
                     key_start + prefix_len
                 } else {
+                    // for invalid UTF-8 we count bytes
                     key_start + remainder.len().min(limit)
                 }
             }
diff --git a/tests/by-util/test_uniq.rs b/tests/by-util/test_uniq.rs
index 6c9107ce527..8e940a743cd 100644
--- a/tests/by-util/test_uniq.rs
+++ b/tests/by-util/test_uniq.rs
@@ -1176,11 +1176,23 @@ fn test_stdin_w1_multibyte() {
     let input = "à\ná\n";
     new_ucmd!()
         .args(&["-w1"])
+        .env("LC_ALL", "en_US.UTF-8")
         .pipe_in(input)
         .succeeds()
         .stdout_is("à\ná\n");
 }
 
+#[test]
+fn test_c_locale_counts_bytes() {
+    let input = "가나다라마\n가나다바사\n";
+    new_ucmd!()
+        .args(&["-w", "4"])
+        .env("LC_ALL", "C")
+        .pipe_in(input)
+        .succeeds()
+        .stdout_is("가나다라마\n");
+}
+
 #[cfg(target_os = "linux")]
 #[test]
 fn test_failed_write_is_reported() {