diff --git a/.github/workflows/test.yaml b/.github/workflows/test.yaml
index fa5e2e5..2fdd92e 100644
--- a/.github/workflows/test.yaml
+++ b/.github/workflows/test.yaml
@@ -77,6 +77,20 @@ jobs:
           ! bazelisk test //:failing_test
           ! bazelisk test //:broken_test
           ! bazelisk test //:empty_test
+      - name: WARNINGS AS ERRORS FAIL
+        working-directory: test
+        run: |
+          # "! cmd" does not stop a "bash -e" step, so test the status
+          # explicitly.
+          for app in unused undef io_warn; do
+            if bazelisk build //warnings:$app 2> warnings_$app.log; then
+              echo "//warnings:$app built despite --warnings-as-errors"
+              exit 1
+            fi
+            cat warnings_$app.log
+            grep -F "Compilation failed due to warnings while using the --warnings-as-errors option" \
+              warnings_$app.log
+          done
       - name: UPLOAD TEST LOGS
         if: always()
         uses: actions/upload-artifact@v4
diff --git a/private/elixir_bytecode.bzl b/private/elixir_bytecode.bzl
index 2359411..3e7acc6 100644
--- a/private/elixir_bytecode.bzl
+++ b/private/elixir_bytecode.bzl
@@ -58,11 +58,26 @@ fi
 {env}
 
 {setup}
-set -x
+diagnostics=$(mktemp)
+status=0
 ${{ABS_ELIXIR_HOME}}/bin/elixirc \\
     -o {out_dir} \\
     {elixirc_opts} \\
-    {srcs}
+    {srcs} 2>"$diagnostics" || status=$?
+cat "$diagnostics" >&2
+# Released Elixir ignores --warnings-as-errors in elixirc and exits 0 on
+# warnings. Fail here instead. Once elixirc fails by itself, status is
+# not 0 and this check does nothing. The launcher's warning about a
+# latin1 file name encoding is not a compiler warning, so it is skipped.
+if [ "$status" = 0 ] && [ -n "{warnings_as_errors}" ] && awk '
+    /^[[:space:]]*warning:/ && !/native name encoding of latin1/ {{ found = 1 }}
+    END {{ exit !found }}
+' "$diagnostics"; then
+    echo "Compilation failed due to warnings while using the --warnings-as-errors option" >&2
+    status=1
+fi
+rm -f "$diagnostics"
+exit "$status"
 """.format(
         maybe_install_erlang = maybe_install_erlang(ctx),
         erlang_home = erlang_home,
@@ -73,6 +88,7 @@ ${{ABS_ELIXIR_HOME}}/bin/elixirc \\
         out_dir = ebin.path,
         elixirc_opts = " ".join([shell.quote(opt) for opt in ctx.attr.elixirc_opts]),
         srcs = " ".join([f.path for f in ctx.files.srcs]),
+        warnings_as_errors = "1" if "--warnings-as-errors" in ctx.attr.elixirc_opts else "",
     )
 
     inputs = depset(
diff --git a/test/warnings/BUILD.bazel b/test/warnings/BUILD.bazel
new file mode 100644
index 0000000..ab9779a
--- /dev/null
+++ b/test/warnings/BUILD.bazel
@@ -0,0 +1,29 @@
+load("@rules_elixir//private:elixir_bytecode.bzl", "elixir_bytecode")
+
+# Each source has one warning. With --warnings-as-errors the build must
+# fail; .github/workflows/test.yaml checks that, so these are manual.
+#   unused:  an unused variable, a compile-time warning
+#   undef:   a call to an undefined remote function, a warning the
+#            compiler emits after the module is compiled
+#   io_warn: IO.warn at compile time, printed without indentation
+[
+    elixir_bytecode(
+        name = name,
+        srcs = [name + ".ex"],
+        dest = name,
+        elixirc_opts = ["--warnings-as-errors"],
+        tags = ["manual"],
+    )
+    for name in [
+        "unused",
+        "undef",
+        "io_warn",
+    ]
+]
+
+# The same warning as :unused, without --warnings-as-errors. It builds.
+elixir_bytecode(
+    name = "allowed",
+    srcs = ["unused.ex"],
+    dest = "allowed",
+)
diff --git a/test/warnings/io_warn.ex b/test/warnings/io_warn.ex
new file mode 100644
index 0000000..57e4b19
--- /dev/null
+++ b/test/warnings/io_warn.ex
@@ -0,0 +1,7 @@
+defmodule Warnings.IoWarn do
+  # A warning printed by code that runs at compile time. Unlike the
+  # compiler's own warnings, it is not indented.
+  IO.warn("a warning from IO.warn at compile time")
+
+  def f, do: :ok
+end
diff --git a/test/warnings/undef.ex b/test/warnings/undef.ex
new file mode 100644
index 0000000..e1d16f6
--- /dev/null
+++ b/test/warnings/undef.ex
@@ -0,0 +1,3 @@
+defmodule Warnings.Undef do
+  def f, do: NoSuchModule.nope()
+end
diff --git a/test/warnings/unused.ex b/test/warnings/unused.ex
new file mode 100644
index 0000000..0d4454d
--- /dev/null
+++ b/test/warnings/unused.ex
@@ -0,0 +1,6 @@
+defmodule Warnings.Unused do
+  def f(x) do
+    y = 1
+    x
+  end
+end