diff --git a/.github/workflows/pr-labeler.yaml b/.github/workflows/pr-labeler.yaml
new file mode 100644
index 000000000..f1b3e9d08
--- /dev/null
+++ b/.github/workflows/pr-labeler.yaml
@@ -0,0 +1,73 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+name: Pull request labeler
+run-name: >-
+  Label pull request ${{github.event.pull_request.number}} by ${{github.actor}}
+
+# This workflow is designed NOT to fail if labeling actions encounter errors;
+# instead, it notes errors as annotations on the workflow's run summary page. If
+# labels don't seem to be getting applied, check there for errors.
+
+on:
+  # Note: do not copy-paste this workflow with `pull_request_target` left as-is.
+  # Its use here is a special case where security implications are understood.
+  # Workflows should normally use `pull_request` instead.
+  pull_request_target:
+    types:
+      - opened
+      - synchronize
+
+  # Allow manual invocation.
+  workflow_dispatch:
+    inputs:
+      pr-number:
+        description: 'The PR number of the PR to label:'
+        type: string
+        required: true
+      debug:
+        description: 'Run with debugging options'
+        type: boolean
+        default: true
+
+# Declare default workflow permissions as read only.
+permissions: read-all
+
+jobs:
+  label-pr-size:
+    if: github.repository_owner == 'quantumlib'
+    name: Update PR size labels
+    runs-on: ubuntu-slim
+    timeout-minutes: 5
+    permissions:
+      contents: read
+      issues: write
+      pull-requests: write
+    env:
+      # Environment variable PR_NUMBER is needed by size-labeler.sh.
+      PR_NUMBER: ${{inputs.pr-number || github.event.pull_request.number}}
+      # Add xtrace to SHELLOPTS for all Bash scripts when doing debug runs.
+      SHELLOPTS: ${{inputs.debug && 'xtrace' || '' }}
+    steps:
+      - name: Check out a copy of the git repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          sparse-checkout: |
+            ./dev_tools/ci/size-labeler.sh
+
+      - name: Label the PR with a size label
+        continue-on-error: true
+        env:
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}
+        run: ./dev_tools/ci/size-labeler.sh
diff --git a/dev_tools/ci/README.md b/dev_tools/ci/README.md
new file mode 100644
index 000000000..49e789ba0
--- /dev/null
+++ b/dev_tools/ci/README.md
@@ -0,0 +1,4 @@
+# Continuous integration scripts
+
+The scripts in this directory are used by the workflows in
+[`../../.github/workflows/`](../../.github/workflows/).
diff --git a/dev_tools/ci/size-labeler.sh b/dev_tools/ci/size-labeler.sh
new file mode 100755
index 000000000..4f0d0405b
--- /dev/null
+++ b/dev_tools/ci/size-labeler.sh
@@ -0,0 +1,224 @@
+#!/usr/bin/env bash
+# Copyright 2025 The Cirq Developers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -euo pipefail -o errtrace
+shopt -s inherit_errexit
+
+declare -r usage="Usage: ${0##*/} [-h | --help | help]
+
+Updates the size labels on a pull request based on the number of lines it
+changes.  The script requires the following environment variables:
+PR_NUMBER, GITHUB_REPOSITORY, GITHUB_TOKEN.  The script is intended
+for automated execution from GitHub Actions workflow."
+
+declare -ar LABELS=(
+    "size: XS"
+    "size: S"
+    "size: M"
+    "size: L"
+    "size: XL"
+)
+
+declare -A LIMITS=(
+    ["${LABELS[0]}"]=10
+    ["${LABELS[1]}"]=50
+    ["${LABELS[2]}"]=200
+    ["${LABELS[3]}"]=800
+    ["${LABELS[4]}"]="$((2 ** 63 - 1))"
+)
+
+declare -ar IGNORED=(
+    "*_pb2.py"
+    "*_pb2.pyi"
+    "*_pb2_grpc.py"
+    ".*.lock"
+    "*.bundle.js"
+)
+
+function info() {
+    echo >&2 "INFO: ${*}"
+}
+
+function error() {
+    echo >&2 "ERROR: ${*}"
+}
+
+function jq_stdin() {
+    local infile
+    infile="$(mktemp)"
+    readonly infile
+    local jq_status=0
+
+    cat >"${infile}"
+    jq_file "$@" "${infile}" || jq_status="${?}"
+    rm "${infile}"
+    return "${jq_status}"
+}
+
+function jq_file() {
+    # Regardless of the success, store the return code.
+    # Prepend each sttderr with command args and send back to stderr.
+    jq "${@}" 2> >(awk -v h="stderr from jq ${*}:" '{print h, $0}' 1>&2) &&
+        rc="${?}" ||
+        rc="${?}"
+    if [[ "${rc}" != "0" ]]; then
+        error "The jq program failed: ${*}"
+        error "Note the quotes above may be wrong. Here was the (possibly empty) input in ${*: -1}:"
+        cat "${@: -1}" # Assumes last argument is input file!!
+    fi
+    return "${rc}"
+}
+
+function api_call() {
+    local -r endpoint="${1// /%20}" # love that our label names have spaces...
+    local -r uri="https://api.github.com/repos/${GITHUB_REPOSITORY}"
+    local response
+    local curl_status=0
+    info "Calling: ${uri}/${endpoint}"
+    response="$(curl -sSL \
+        --fail-with-body \
+        --connect-timeout 10 --max-time 20 \
+        -H "Authorization: token ${GITHUB_TOKEN}" \
+        -H "Accept: application/vnd.github.v3.json" \
+        -H "X-GitHub-Api-Version:2022-11-28" \
+        -H "Content-Type: application/json" \
+        "${@:2}" \
+        "${uri}/${endpoint}"
+    )" || curl_status="${?}"
+    if [[ -n "${response}" ]]; then
+        cat <<<"${response}"
+    fi
+    if (( curl_status )); then
+        error "GitHub API call failed (curl exit $curl_status) for ${uri}/${endpoint}"
+        error "Response body:"
+        cat >&2 <<<"${response}"
+    fi
+    return "${curl_status}"
+}
+
+function compute_changes() {
+    local -r pr="$1"
+    local page=1
+    local total_changes=0
+    while true; do
+        local response
+        response="$(api_call "pulls/${pr}/files?per_page=100&page=${page}")"
+
+        if [[ "$(jq_stdin '. | length' <<<"${response}")" -eq 0 ]]; then
+            break
+        fi
+
+        local name changes
+        while IFS= read -r name && IFS= read -r changes; do
+            for pattern in "${IGNORED[@]}"; do
+                # shellcheck disable=SC2053  # Need leave the pattern unquoted.
+                if [[ "${name}" == ${pattern} ]]; then
+                    info "File ${name} ignored"
+                    continue 2
+                fi
+            done
+            info "File ${name} +-${changes}"
+            total_changes="$((total_changes + changes))"
+        done < <(jq_stdin -r '.[] | .filename, .changes' <<<"${response}")
+        ((page++))
+    done
+    echo "${total_changes}"
+}
+
+function get_size_label() {
+    local -r changes="$1"
+    for label in "${LABELS[@]}"; do
+        local limit="${LIMITS["${label}"]}"
+        if [[ "${changes}" -lt "${limit}" ]]; then
+            echo "${label}"
+            return
+        fi
+    done
+}
+
+function prune_stale_labels() {
+    local -r pr="$1"
+    local -r size_label="$2"
+    local response
+    local existing_labels
+    response="$(api_call "pulls/${pr}")"
+    existing_labels="$(jq_stdin -r '.labels[] | .name' <<<"${response}")"
+    readarray -t existing_labels <<<"${existing_labels}"
+
+    local correctly_labeled=false
+    for label in "${existing_labels[@]}"; do
+        [[ -z "${label}" ]] && continue
+        # If the label we want is already present, we can just leave it there.
+        if [[ "${label}" == "${size_label}" ]]; then
+            info "Label '${label}' is correct, leaving it."
+            correctly_labeled=true
+            continue
+        fi
+        # If there is another size label, we need to remove it
+        if [[ -v "LIMITS[${label}]" ]]; then
+            info "Label '${label}' is stale, removing it."
+            api_call "issues/${pr}/labels/${label}" -X DELETE >/dev/null
+            continue
+        fi
+        info "Label '${label}' is unknown, leaving it."
+    done
+    echo "${correctly_labeled}"
+}
+
+function main() {
+    local moreinfo="(Use --help option for more info.)"
+    if (( $# )); then
+        case "$1" in
+            -h | --help | help)
+                echo "$usage"
+                exit 0
+                ;;
+            *)
+                error "Invalid argument '$1'.  ${moreinfo}"
+                exit 2
+                ;;
+        esac
+    fi
+    local env_var_name
+    local env_var_missing=0
+    for env_var_name in PR_NUMBER GITHUB_TOKEN GITHUB_REPOSITORY; do
+        if [[ ! -v "${env_var_name}" ]]; then
+            env_var_missing=1
+            error "Missing environment variable ${env_var_name}"
+        fi
+    done
+    if (( env_var_missing )); then
+        error "${moreinfo}"
+        exit 2
+    fi
+
+    local total_changes
+    total_changes="$(compute_changes "$PR_NUMBER")"
+    info "Lines changed: ${total_changes}"
+
+    local size_label
+    size_label="$(get_size_label "$total_changes")"
+    info "Appropriate label is '${size_label}'"
+
+    local correctly_labeled
+    correctly_labeled="$(prune_stale_labels "$PR_NUMBER" "${size_label}")"
+
+    if [[ "${correctly_labeled}" != true ]]; then
+        api_call "issues/$PR_NUMBER/labels" -X POST -d "{\"labels\":[\"${size_label}\"]}" >/dev/null
+        info "Added label '${size_label}'"
+    fi
+}
+
+main "$@"