diff --git a/.github/scripts/notify-app-build.mjs b/.github/scripts/notify-app-build.mjs
new file mode 100644
index 0000000000..6bb743b826
--- /dev/null
+++ b/.github/scripts/notify-app-build.mjs
@@ -0,0 +1,122 @@
+const {
+	BUILD_ENVIRONMENT,
+	GITHUB_API_URL,
+	GITHUB_REF_NAME,
+	GITHUB_REPOSITORY,
+	GITHUB_RUN_ID,
+	GITHUB_SERVER_URL,
+	GITHUB_SHA,
+	GITHUB_TOKEN,
+	GITHUB_TRIGGERING_ACTOR,
+	SLACK_WEBHOOK_URL,
+} = process.env
+
+/** Job and artifact names must match the matrix in `.github/workflows/theseus-build.yml`. */
+const PLATFORMS = [
+	{
+		label: 'Windows',
+		job: 'Build (namespace-profile-medium-amd64-windows)',
+		artifact: 'App bundle (x86_64-pc-windows-msvc)',
+	},
+	{
+		label: 'MacOS',
+		job: 'Build (namespace-profile-medium-amd64-macos)',
+		artifact: 'App bundle (universal-apple-darwin)',
+	},
+	{
+		label: 'Linux',
+		job: 'Build (namespace-profile-medium-amd64)',
+		artifact: 'App bundle (x86_64-unknown-linux-gnu)',
+	},
+]
+
+const runUrl = `${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}/actions/runs/${GITHUB_RUN_ID}`
+
+async function fetchRunResource(resource) {
+	const response = await fetch(
+		`${GITHUB_API_URL}/repos/${GITHUB_REPOSITORY}/actions/runs/${GITHUB_RUN_ID}/${resource}?per_page=100`,
+		{
+			headers: {
+				Accept: 'application/vnd.github+json',
+				Authorization: `Bearer ${GITHUB_TOKEN}`,
+			},
+		},
+	)
+	if (!response.ok) {
+		throw new Error(`Failed to fetch ${resource}: ${response.status} ${await response.text()}`)
+	}
+	return response.json()
+}
+
+/** Slack treats these characters as control sequences, even inside inline code. */
+function escapeMrkdwn(text) {
+	return text.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;')
+}
+
+function mrkdwnSection(text) {
+	return { type: 'section', text: { type: 'mrkdwn', text } }
+}
+
+function mrkdwnContext(text) {
+	return { type: 'context', elements: [{ type: 'mrkdwn', text }] }
+}
+
+/** A build without an artifact is reported as failed, as there is nothing to download. */
+function summarizeBuild({ label, job: jobName, artifact: artifactName }, jobs, artifacts) {
+	const job = jobs.find(({ name }) => name === jobName)
+	if (!job) {
+		throw new Error(`No job named "${jobName}" in this run`)
+	}
+
+	const artifact = artifacts.find(({ name }) => name === artifactName)
+	if (job.conclusion !== 'success' || !artifact) {
+		const failedStep = job.steps.find(({ conclusion }) => conclusion === 'failure')
+		const logsUrl = failedStep ? `${job.html_url}#step:${failedStep.number}:1` : job.html_url
+		return { succeeded: false, line: `:x: ${label} build *FAILED* (<${logsUrl}|view logs>)` }
+	}
+
+	const sizeMiB = Math.round(artifact.size_in_bytes / 1024 / 1024)
+	const downloadUrl = `${runUrl}/artifacts/${artifact.id}`
+	return {
+		succeeded: true,
+		line: `:white_check_mark: ${label} build *SUCCESS* (<${downloadUrl}|download> ${sizeMiB}MiB)`,
+	}
+}
+
+const [{ jobs }, { artifacts }] = await Promise.all([
+	fetchRunResource('jobs'),
+	fetchRunResource('artifacts'),
+])
+
+const builds = PLATFORMS.map((platform) => summarizeBuild(platform, jobs, artifacts))
+const statusEmoji = builds.every(({ succeeded }) => succeeded) ? ':white_check_mark:' : ':x:'
+const headline = `A new Modrinth app build has been completed ${statusEmoji}`
+
+const now = new Date()
+const timestamp = `<!date^${Math.floor(now.getTime() / 1000)}^{date_short_pretty} at {time}|${now.toUTCString()}>`
+
+const response = await fetch(SLACK_WEBHOOK_URL, {
+	method: 'POST',
+	headers: { 'Content-Type': 'application/json' },
+	body: JSON.stringify({
+		text: headline,
+		blocks: [
+			mrkdwnSection(`:modrinth: ${headline}`),
+			mrkdwnContext(
+				[
+					`Environment: \`${BUILD_ENVIRONMENT}\``,
+					`Branch: \`${escapeMrkdwn(GITHUB_REF_NAME)}\``,
+					`Commit: \`${GITHUB_SHA.slice(0, 8)}\``,
+					`Manually run by <${GITHUB_SERVER_URL}/${GITHUB_TRIGGERING_ACTOR}|${GITHUB_TRIGGERING_ACTOR}>`,
+				].join('\n'),
+			),
+			mrkdwnSection(builds.map(({ line }) => line).join('\n')),
+			{ type: 'divider' },
+			mrkdwnContext(`<${runUrl}|View on GitHub> • ${timestamp}`),
+		],
+	}),
+})
+
+if (!response.ok) {
+	throw new Error(`Failed to post to Slack: ${response.status} ${await response.text()}`)
+}
diff --git a/.github/workflows/theseus-build.yml b/.github/workflows/theseus-build.yml
index d4903a4cbb..cd8b5188a8 100644
--- a/.github/workflows/theseus-build.yml
+++ b/.github/workflows/theseus-build.yml
@@ -225,3 +225,24 @@ jobs:
             target/release/bundle/nsis/Modrinth App_*-setup.exe*
             target/release/bundle/nsis/Modrinth App (dev-*)_*-setup.exe*
             target/release/bundle/nsis/Modrinth App_*-setup.nsis.zip*
+
+  notify:
+    name: Notify Slack
+    needs: build
+    if: ${{ !cancelled() && github.event_name == 'workflow_dispatch' }}
+    runs-on: ubuntu-latest
+    permissions:
+      actions: read
+      contents: read
+    steps:
+      - name: Check out code
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          sparse-checkout: .github/scripts
+
+      - name: Send Slack notification
+        run: node .github/scripts/notify-app-build.mjs
+        env:
+          BUILD_ENVIRONMENT: ${{ inputs.environment || 'prod' }}
+          GITHUB_TOKEN: ${{ github.token }}
+          SLACK_WEBHOOK_URL: ${{ secrets.SLACK_APP_BUILDS_WEBHOOK_URL }}