diff --git a/rules/lock.bzl b/rules/lock.bzl
index 4079ead..9c9a822 100644
--- a/rules/lock.bzl
+++ b/rules/lock.bzl
@@ -27,7 +27,6 @@ def _pip_lock_impl(ctx):
         "@@USE_PY2@@": "true" if "2" in ctx.attr.python_version else "false",
         "@@USE_PY3@@": "true" if "3" in ctx.attr.python_version else "false",
         "@@WHEEL_DIRECTORY@@": wheel_dir,
-        "@@INDEX_URL@@": ctx.attr.index_url,
     }
 
     ctx.actions.expand_template(
@@ -55,14 +54,14 @@ pip_lock = rule(
     doc = """
         Defines a binary target that may be executed via `bazel run` in order to compile
         any number of `requirements.txt` files into a single `requirements-lock.json`
-        file. This binary should be executed on all supported platforms to add the 
-        correct set of requirements to the lock file for each platform. 
+        file. This binary should be executed on all supported platforms to add the
+        correct set of requirements to the lock file for each platform.
     """,
     attrs = {
         "requirements": attr.label_list(
             allow_files = True,
             doc = """
-                Files following the standard requirements file format 
+                Files following the standard requirements file format
                 (https://pip.pypa.io/en/stable/reference/pip_install/#requirements-file-format)
 
                 These should define direct dependencies only, and should not pin
@@ -73,7 +72,7 @@ pip_lock = rule(
             default = "requirements-lock.json",
             doc = """
                 A path relative to the package in which this rule is defined to which
-                the compiled lock file should be written. This file should 
+                the compiled lock file should be written. This file should
                 be source-controlled and provided as input to the `pip_repository` rule.
             """,
         ),
@@ -82,7 +81,7 @@ pip_lock = rule(
             default = "PY2AND3",
             doc = """
                 The Python versions for which to compile the requirement set. The
-                requirements lock file will contain one environment per Python version 
+                requirements lock file will contain one environment per Python version
                 per platform. Each environment will define its locked requirement set.
             """,
         ),
@@ -91,20 +90,16 @@ pip_lock = rule(
             doc = """
                 A path to a directory relative to the package in which this rule is
                 defined in which built wheels will be stored. If the given directory
-                does not already exist, it will be created. 
-                
-                Wheels will be built for any required distribution that is not already 
+                does not already exist, it will be created.
+
+                Wheels will be built for any required distribution that is not already
                 available as a wheel for the given environment. These wheels may be
-                committed to source control or published to a custom Python package 
+                committed to source control or published to a custom Python package
                 index. If the latter approach is used, this binary should be run again
-                with the `index_url` attribute set to the URL of that index in order to 
+                with the `--index-url` argument set to the URL of that index in order to
                 resolve the new locations of those wheels.
             """,
         ),
-        "index_url": attr.string(
-            default = "https://pypi.org/simple",
-            doc = "The URL of a custom Python package index to use instead of PyPI.",
-        ),
         "_lock_pip_requirements_py2": attr.label(
             default = "//src/bin:lock_pip_requirements_py2",
             cfg = "target",
diff --git a/src/bin/lock_pip_requirements.py b/src/bin/lock_pip_requirements.py
index f09d673..ae92d6f 100644
--- a/src/bin/lock_pip_requirements.py
+++ b/src/bin/lock_pip_requirements.py
@@ -18,6 +18,7 @@ def main():
     workspace_directory = get_workspace_directory()
     lock_file_path = os.path.join(workspace_directory, args.lock_file_path)
     wheel_directory = os.path.join(workspace_directory, args.wheel_dir)
+    index_urls = args.index_urls or ["https://pypi.org/simple"]
 
     LOG.info("Locking pip requirements for Python %s", sys.version_info.major)
 
@@ -36,7 +37,7 @@ def main():
     resolved_requirements = resolve.resolve_requirement_set(
         requirement_set,
         pip_session,
-        [args.index_url],
+        index_urls,
         wheel_directory,
     )
 
@@ -87,7 +88,8 @@ def parse_args():
     )
     parser.add_argument(
         "-i", "--index-url",
-        default="https://pypi.org/simple",
+        action="append",
+        dest="index_urls",
     )
     parser.add_argument(
         "-w", "--wheel-dir",
diff --git a/src/templates/lock_pip_requirements_wrapper_template.sh b/src/templates/lock_pip_requirements_wrapper_template.sh
index 5f4f35b..e1f4691 100644
--- a/src/templates/lock_pip_requirements_wrapper_template.sh
+++ b/src/templates/lock_pip_requirements_wrapper_template.sh
@@ -10,7 +10,6 @@ REQUIREMENTS_LOCK_PATH="@@REQUIREMENTS_LOCK_PATH@@"
 USE_PY2=@@USE_PY2@@
 USE_PY3=@@USE_PY3@@
 WHEEL_DIRECTORY="@@WHEEL_DIRECTORY@@"
-INDEX_URL=@@INDEX_URL@@
 
 REQUESTING_HELP=0
 for arg in $@; do
@@ -23,7 +22,6 @@ if [ "$USE_PY2" = true ]; then
     $LOCK_PIP_REQUIREMENTS_PY2 \
         --lock-file $REQUIREMENTS_LOCK_PATH \
         --wheel-dir $WHEEL_DIRECTORY \
-        --index-url $INDEX_URL \
         --workspace-name $WORKSPACE_NAME \
         "$@" \
         $REQUIREMENTS_TXT_PATHS
@@ -37,7 +35,6 @@ if [ "$USE_PY3" = true ]; then
     $LOCK_PIP_REQUIREMENTS_PY3 \
         --lock-file $REQUIREMENTS_LOCK_PATH \
         --wheel-dir $WHEEL_DIRECTORY \
-        --index-url $INDEX_URL \
         --workspace-name $WORKSPACE_NAME \
         "$@" \
         $REQUIREMENTS_TXT_PATHS