diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ffaa2b03df9e7..aa9d97ba477b7 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -72,6 +72,9 @@ jobs:
       - name: decide whether to skip this job
         run: src/ci/scripts/should-skip-this.sh
         if: success() && !env.SKIP_JOB
+      - name: ensure the channel matches the target branch
+        run: src/ci/scripts/verify-channel.sh
+        if: success() && !env.SKIP_JOB
       - name: configure GitHub Actions to kill the build when outdated
         uses: rust-lang/simpleinfra/github-actions/cancel-outdated-builds@master
         with:
@@ -434,6 +437,9 @@ jobs:
       - name: decide whether to skip this job
         run: src/ci/scripts/should-skip-this.sh
         if: success() && !env.SKIP_JOB
+      - name: ensure the channel matches the target branch
+        run: src/ci/scripts/verify-channel.sh
+        if: success() && !env.SKIP_JOB
       - name: configure GitHub Actions to kill the build when outdated
         uses: rust-lang/simpleinfra/github-actions/cancel-outdated-builds@master
         with:
@@ -541,6 +547,9 @@ jobs:
       - name: decide whether to skip this job
         run: src/ci/scripts/should-skip-this.sh
         if: success() && !env.SKIP_JOB
+      - name: ensure the channel matches the target branch
+        run: src/ci/scripts/verify-channel.sh
+        if: success() && !env.SKIP_JOB
       - name: configure GitHub Actions to kill the build when outdated
         uses: rust-lang/simpleinfra/github-actions/cancel-outdated-builds@master
         with:
diff --git a/src/ci/channel b/src/ci/channel
new file mode 100644
index 0000000000000..bf867e0ae5b6c
--- /dev/null
+++ b/src/ci/channel
@@ -0,0 +1 @@
+nightly
diff --git a/src/ci/github-actions/ci.yml b/src/ci/github-actions/ci.yml
index a59a90b86bcaf..343091cb779fc 100644
--- a/src/ci/github-actions/ci.yml
+++ b/src/ci/github-actions/ci.yml
@@ -126,6 +126,10 @@ x--expand-yaml-anchors--remove:
         run: src/ci/scripts/should-skip-this.sh
         <<: *step
 
+      - name: ensure the channel matches the target branch
+        run: src/ci/scripts/verify-channel.sh
+        <<: *step
+
       - name: configure GitHub Actions to kill the build when outdated
         uses: rust-lang/simpleinfra/github-actions/cancel-outdated-builds@master
         with:
diff --git a/src/ci/run.sh b/src/ci/run.sh
index 1958b6ee41d7f..35f80a935c693 100755
--- a/src/ci/run.sh
+++ b/src/ci/run.sh
@@ -62,17 +62,10 @@ if [ "$DIST_SRC" = "" ]; then
   RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --disable-dist-src"
 fi
 
-# If we're deploying artifacts then we set the release channel, otherwise if
-# we're not deploying then we want to be sure to enable all assertions because
-# we'll be running tests
-#
-# FIXME: need a scheme for changing this `nightly` value to `beta` and `stable`
-#        either automatically or manually.
-export RUST_RELEASE_CHANNEL=nightly
-
 # Always set the release channel for bootstrap; this is normally not important (i.e., only dist
 # builds would seem to matter) but in practice bootstrap wants to know whether we're targeting
 # master, beta, or stable with a build to determine whether to run some checks (notably toolstate).
+export RUST_RELEASE_CHANNEL="$(cat "${ci_dir}/channel")"
 RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --release-channel=$RUST_RELEASE_CHANNEL"
 
 if [ "$DEPLOY$DEPLOY_ALT" = "1" ]; then
diff --git a/src/ci/scripts/verify-channel.sh b/src/ci/scripts/verify-channel.sh
new file mode 100755
index 0000000000000..d02dc362c63b6
--- /dev/null
+++ b/src/ci/scripts/verify-channel.sh
@@ -0,0 +1,38 @@
+#!/bin/bash
+# We want to make sure all PRs are targeting the right branch when they're
+# opened, otherwise we risk (for example) to land a beta-specific change to the
+# master branch. This script ensures the branch of the PR matches the channel.
+
+set -euo pipefail
+IFS=$'\n\t'
+
+source "$(cd "$(dirname "$0")" && pwd)/../shared.sh"
+
+if isCiBranch auto || isCiBranch try; then
+    echo "channel verification is only executed on PR builds"
+    exit
+fi
+
+channel=$(cat "$(ciCheckoutPath)/src/ci/channel")
+case "${channel}" in
+    nightly)
+        channel_branch="master"
+        ;;
+    beta)
+        channel_branch="beta"
+        ;;
+    stable)
+        channel_branch="stable"
+        ;;
+    *)
+        echo "error: unknown channel defined in src/ci/channel: ${channel}"
+        exit 1
+esac
+
+branch="$(ciBaseBranch)"
+if [[ "${branch}" != "${channel_branch}" ]]; then
+    echo "error: PRs changing the \`${channel}\` channel should be sent to the \
+\`${channel_branch}\` branch!"
+
+    exit 1
+fi
diff --git a/src/ci/shared.sh b/src/ci/shared.sh
index 3c196c9478cf8..332a949a4dc51 100644
--- a/src/ci/shared.sh
+++ b/src/ci/shared.sh
@@ -73,6 +73,18 @@ function isCiBranch {
     fi
 }
 
+function ciBaseBranch {
+    if isAzurePipelines; then
+        echo "unsupported on Azure Pipelines"
+        exit 1
+    elif isGitHubActions; then
+        echo "${GITHUB_BASE_REF#refs/heads/}"
+    else
+        echo "ciBaseBranch only works inside CI!"
+        exit 1
+    fi
+}
+
 function ciCommit {
     if isAzurePipelines; then
         echo "${BUILD_SOURCEVERSION}"