diff --git a/examples/file-replace.roc b/examples/file-replace.roc
new file mode 100644
index 00000000..e8f5578f
--- /dev/null
+++ b/examples/file-replace.roc
@@ -0,0 +1,27 @@
+## Replace every occurrence of a substring in a UTF-8 file in place.
+app [main!] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.21.0-rc4/FvCh4vdqm3nBY6DWEfZ8RuGCVfjuMY43HA8KSNk9qVDn.tar.zst" }
+
+import pf.OsStr
+import pf.Stdout
+import pf.Path
+
+main! : List(OsStr) => Try({}, _)
+main! = |_args| {
+
+	file : Path
+	file = "greeting.txt"
+
+	file.write_utf8!("Hello, World! Hello, Roc!")?
+
+	# Replaces both occurrences of "Hello", not just the first.
+	file.replace_utf8!("Hello", "Goodbye")?
+
+	contents = file.read_utf8!()?
+
+	# Cleanup
+	file.delete!()?
+
+	Stdout.line!("After replacing: \"${contents}\"")?
+
+	Ok({})
+}
diff --git a/platform/Path.roc b/platform/Path.roc
index 982be245..6ade3a6d 100644
--- a/platform/Path.roc
+++ b/platform/Path.roc
@@ -107,6 +107,18 @@ Path := [
 	write_utf8! : Path, Str => Try({}, [PathErr(IOErr), ..])
 	write_utf8! = |path, content| map_file_result(Host.file_write_utf8!(to_raw(path), content))
 
+	## Replace every occurrence of `pattern` with `replacement` in the UTF-8 file
+	## at this path.
+	##
+	## This reads the whole file, substitutes, and writes it back, so it is not
+	## atomic: a failure mid-write can leave the file partially written, exactly
+	## as a bare [write_utf8!] would.
+	replace_utf8! : Path, Str, Str => Try({}, [PathErr(IOErr), ..])
+	replace_utf8! = |path, pattern, replacement| {
+		content = read_utf8!(path)?
+		write_utf8!(path, Str.replace_each(content, pattern, replacement))
+	}
+
 	## Delete a file at this path.
 	delete! : Path => Try({}, [PathErr(IOErr), ..])
 	delete! = |path| map_file_result(Host.file_delete!(to_raw(path)))
diff --git a/scripts/test_spec.json b/scripts/test_spec.json
index 25789463..9377f724 100644
--- a/scripts/test_spec.json
+++ b/scripts/test_spec.json
@@ -148,6 +148,18 @@
         }
       ]
     },
+    {
+      "path": "examples/file-replace.roc",
+      "cases": [
+        {
+          "name": "happy",
+          "temp_cwd": true,
+          "contains": [
+            "After replacing: \"Goodbye, World! Goodbye, Roc!\""
+          ]
+        }
+      ]
+    },
     {
       "path": "examples/file-size.roc",
       "cases": [