diff --git a/src/uu/uniq/src/uniq.rs b/src/uu/uniq/src/uniq.rs
index 6f324929cc3..f992d17182d 100644
--- a/src/uu/uniq/src/uniq.rs
+++ b/src/uu/uniq/src/uniq.rs
@@ -836,6 +836,21 @@ fn open_input_file(in_file_name: Option<&OsStr>) -> UResult<Box<dyn BufRead>> {
     })
 }
 
+fn stdbuf_writer<W: Write + 'static>(writer: W) -> Box<dyn Write> {
+    match std::env::var_os("_STDBUF_O") {
+        Some(v) if v == "L" => Box::new(std::io::LineWriter::new(writer)),
+        Some(v) => {
+            // TODO: verify parsing error
+            let capacity = v
+                .to_str()
+                .and_then(|s| s.parse().ok())
+                .unwrap_or(OUTPUT_BUFFER_CAPACITY);
+            Box::new(BufWriter::with_capacity(capacity, writer))
+        }
+        None => Box::new(BufWriter::with_capacity(OUTPUT_BUFFER_CAPACITY, writer)),
+    }
+}
+
 // None or "-" means stdout.
 fn open_output_file(out_file_name: Option<&OsStr>) -> UResult<Box<dyn Write>> {
     Ok(match out_file_name {
@@ -843,11 +858,8 @@ fn open_output_file(out_file_name: Option<&OsStr>) -> UResult<Box<dyn Write>> {
             let out_file = File::create(path).map_err_context(
                 || translate!("uniq-error-could-not-open", "path" => path.maybe_quote()),
             )?;
-            Box::new(BufWriter::with_capacity(OUTPUT_BUFFER_CAPACITY, out_file))
+            stdbuf_writer(out_file)
         }
-        _ => Box::new(BufWriter::with_capacity(
-            OUTPUT_BUFFER_CAPACITY,
-            stdout().lock(),
-        )),
+        _ => stdbuf_writer(stdout().lock()),
     })
 }
diff --git a/tests/by-util/test_uniq.rs b/tests/by-util/test_uniq.rs
index 8e940a743cd..466b9d9c98f 100644
--- a/tests/by-util/test_uniq.rs
+++ b/tests/by-util/test_uniq.rs
@@ -2,11 +2,20 @@
 //
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
+// spell-checker:ignore (vars) STDBUF
 
 // spell-checker:ignore nabcd badoption schar
+use rstest::rstest;
+use std::fs::{File, OpenOptions};
+use std::io::Write;
+use std::process::{Command, Stdio};
+use std::thread;
+use std::time::Duration;
 use uucore::posix::OBSOLETE;
 use uutests::at_and_ucmd;
 use uutests::new_ucmd;
+use uutests::util::TestScenario;
+use uutests::util_name;
 
 static INPUT: &str = "sorted.txt";
 static OUTPUT: &str = "sorted-output.txt";
@@ -1199,7 +1208,59 @@ fn test_failed_write_is_reported() {
     new_ucmd!()
         .pipe_in("hello")
         .args(&["-z"])
-        .set_stdout(std::fs::File::create("/dev/full").unwrap())
+        .set_stdout(File::create("/dev/full").unwrap())
         .fails()
         .stderr_is("uniq: write error: No space left on device\n");
 }
+
+#[cfg(unix)]
+#[rstest]
+#[case(None,        &["1\n", "2\n", "3\n"],  "1\n2\n3\n")]
+#[case(Some("L"),   &["1\n", "2\n", "3\n"],  "1\n")]
+#[case(Some("0"),   &["1\n", "2\n", "3\n"],  "1\n")]
+// TODO: comment out because this is not super reliable i believe
+#[case(Some("128"), &["000000000000000000000000000000000000000000000000000000000000001\n",
+"000000000000000000000000000000000000000000000000000000000000002\n",
+"000000000000000000000000000000000000000000000000000000000000003\n",
+"000000000000000000000000000000000000000000000000000000000000004\n",
+"000000000000000000000000000000000000000000000000000000000000005\n"],
+"000000000000000000000000000000000000000000000000000000000000001\n000000000000000000000000000000000000000000000000000000000000002\n")]
+fn test_uniq_stdbuf_output_mode(
+    #[case] stdbuf_o: Option<&str>,
+    #[case] input: &[&str],
+    #[case] expected_read: &str,
+) {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+    let fifo = at.plus("fifo");
+    let out = at.plus("out");
+    at.mkfifo("fifo");
+    let mut dd = Command::new("dd")
+        .args(["count=1", &format!("if={}", fifo.display())])
+        .stdout(File::create(&out).unwrap())
+        .stderr(Stdio::null())
+        .spawn()
+        .unwrap();
+    let mut cmd = Command::new(&ts.bin_path);
+    if let Some(mode) = stdbuf_o {
+        cmd.env("_STDBUF_O", mode);
+    }
+    let mut uniq = cmd
+        .arg(util_name!())
+        .stdin(Stdio::piped())
+        .stdout(Stdio::from(
+            OpenOptions::new().write(true).open(&fifo).unwrap(),
+        ))
+        .spawn()
+        .unwrap();
+    {
+        let mut stdin = uniq.stdin.take().unwrap();
+        for line in input {
+            stdin.write_all(line.as_bytes()).unwrap();
+            thread::sleep(Duration::from_millis(100));
+        }
+    }
+    let _ = dd.wait();
+    let _ = uniq.wait();
+    assert_eq!(std::fs::read_to_string(&out).unwrap(), expected_read);
+}