diff --git a/cli/src/format.rs b/cli/src/format.rs
index 8ced266bce..f5a32c9caa 100644
--- a/cli/src/format.rs
+++ b/cli/src/format.rs
@@ -1,5 +1,6 @@
 use std::path::PathBuf;
 
+use crate::FormatMode;
 use bumpalo::collections::Vec;
 use bumpalo::Bump;
 use roc_error_macros::{internal_error, user_error};
@@ -24,7 +25,7 @@ use roc_parse::{
 };
 use roc_region::all::{Loc, Region};
 
-pub fn format(files: std::vec::Vec<PathBuf>) {
+pub fn format(files: std::vec::Vec<PathBuf>, mode: FormatMode) -> Result<(), String> {
     for file in files {
         let arena = Bump::new();
 
@@ -99,9 +100,22 @@ pub fn format(files: std::vec::Vec<PathBuf>) {
                 unstable_2_file.display());
         }
 
-        // If all the checks above passed, actually write out the new file.
-        std::fs::write(&file, buf.as_str()).unwrap();
+        match mode {
+            FormatMode::CheckOnly => {
+                // If we notice that this file needs to be formatted, return early
+                if buf.as_str() != src {
+                    return Err("One or more files need to be reformatted.".to_string());
+                }
+            }
+
+            FormatMode::Format => {
+                // If all the checks above passed, actually write out the new file.
+                std::fs::write(&file, buf.as_str()).unwrap();
+            }
+        }
     }
+
+    Ok(())
 }
 
 #[derive(Debug, PartialEq)]
diff --git a/cli/src/lib.rs b/cli/src/lib.rs
index 136f99eab7..e3027927ea 100644
--- a/cli/src/lib.rs
+++ b/cli/src/lib.rs
@@ -37,6 +37,7 @@ pub const FLAG_TIME: &str = "time";
 pub const FLAG_LINK: &str = "roc-linker";
 pub const FLAG_PRECOMPILED: &str = "precompiled-host";
 pub const FLAG_VALGRIND: &str = "valgrind";
+pub const FLAG_CHECK: &str = "check";
 pub const ROC_FILE: &str = "ROC_FILE";
 pub const ROC_DIR: &str = "ROC_DIR";
 pub const BACKEND: &str = "BACKEND";
@@ -122,6 +123,12 @@ pub fn build_app<'a>() -> App<'a> {
                     .index(1)
                     .multiple_values(true)
                     .required(false))
+            .arg(
+                Arg::new(FLAG_CHECK)
+                    .long(FLAG_CHECK)
+                    .about("Checks that specified files are formatted. If formatting is needed, it will return a non-zero exit code.")
+                    .required(false),
+            )
         )
         .subcommand(App::new(CMD_VERSION)
             .about("Print version information")
@@ -242,6 +249,11 @@ pub enum BuildConfig {
     BuildAndRun { roc_file_arg_index: usize },
 }
 
+pub enum FormatMode {
+    Format,
+    CheckOnly,
+}
+
 pub fn build(matches: &ArgMatches, config: BuildConfig) -> io::Result<i32> {
     use build::build_file;
     use std::str::FromStr;
diff --git a/cli/src/main.rs b/cli/src/main.rs
index 7fba8781c5..a90ca61554 100644
--- a/cli/src/main.rs
+++ b/cli/src/main.rs
@@ -1,7 +1,7 @@
 use roc_cli::build::check_file;
 use roc_cli::{
-    build_app, docs, format, BuildConfig, CMD_BUILD, CMD_CHECK, CMD_DOCS, CMD_EDIT, CMD_FORMAT,
-    CMD_REPL, CMD_VERSION, DIRECTORY_OR_FILES, FLAG_TIME, ROC_FILE,
+    build_app, docs, format, BuildConfig, FormatMode, CMD_BUILD, CMD_CHECK, CMD_DOCS, CMD_EDIT,
+    CMD_FORMAT, CMD_REPL, CMD_VERSION, DIRECTORY_OR_FILES, FLAG_CHECK, FLAG_TIME, ROC_FILE,
 };
 use roc_load::file::LoadingProblem;
 use std::fs::{self, FileType};
@@ -150,9 +150,20 @@ fn main() -> io::Result<()> {
                 roc_files_recursive(os_str.as_os_str(), metadata.file_type(), &mut roc_files)?;
             }
 
-            format(roc_files);
+            let format_mode = match matches.is_present(FLAG_CHECK) {
+                true => FormatMode::CheckOnly,
+                false => FormatMode::Format,
+            };
 
-            Ok(0)
+            let format_exit_code = match format(roc_files, format_mode) {
+                Ok(_) => 0,
+                Err(message) => {
+                    eprintln!("{}", message);
+                    1
+                }
+            };
+
+            Ok(format_exit_code)
         }
         Some((CMD_VERSION, _)) => {
             println!("roc {}", concatcp!(include_str!("../../version.txt"), "\n"));