--- a/testing/docker/desktop-build/bin/build.sh
+++ b/testing/docker/desktop-build/bin/build.sh
@@ -1,29 +1,36 @@
 #! /bin/bash -vex
 
 set -x -e -v
 
+# Relative path to in-tree script
+: JOB_SCRIPT                ${JOB_SCRIPT:=testing/taskcluster/scripts/builder/build-linux.sh}
+
+script_args="${@}"
+
 # TODO: when bug 1093833 is solved and tasks can run as non-root, reduce this
 # to a simple fail-if-root check
 if [ $(id -u) = 0 ]; then
     # each of the caches we have mounted are owned by root, so update that ownership
     # to 'worker'
     for cache in /home/worker/.tc-vcs /home/worker/workspace /home/worker/tooltool-cache; do
         if [ -d $cache ]; then
             # -R probably isn't necessary forever, but it fixes some poisoned
             # caches for now
             chown -R worker:worker $cache
         fi
     done
 
     # ..then drop privileges by re-running this script
-    exec su worker /home/worker/bin/build.sh
+    exec su worker -c "/home/worker/bin/build.sh $script_args"
 fi
 
 ####
 # The default build works for any fx_desktop_build based mozharness job:
-# via linux-build.sh
+# via build-linux.sh
 ####
 
 . $HOME/bin/checkout-sources.sh
 
-. $WORKSPACE/build/src/testing/taskcluster/scripts/builder/build-linux.sh
+script=$WORKSPACE/build/src/$JOB_SCRIPT
+chmod +x $script
+exec $script $script_args

new file mode 100644
--- /dev/null
+++ b/testing/taskcluster/scripts/tester/harness-test-linux.sh
@@ -0,0 +1,40 @@
+#! /bin/bash -vex
+
+set -x -e
+
+echo "running as" $(id)
+
+####
+# Taskcluster friendly wrapper for running a script in
+# testing/mozharness/scripts in a source checkout (no build).
+# Example use: Python-only harness unit tests
+####
+
+: WORKSPACE                     ${WORKSPACE:=/home/worker/workspace}
+: SRC_ROOT                      ${SRC_ROOT:=$WORKSPACE/build/src}
+# These paths should be relative to $SRC_ROOT
+: MOZHARNESS_SCRIPT             ${MOZHARNESS_SCRIPT}
+: MOZHARNESS_CONFIG             ${MOZHARNESS_CONFIG}
+: mozharness args               "${@}"
+
+set -v
+cd $WORKSPACE
+
+fail() {
+    echo # make sure error message is on a new line
+    echo "[harness-test-linux.sh:error]" "${@}"
+    exit 1
+}
+
+if [[ -z ${MOZHARNESS_SCRIPT} ]]; then fail "MOZHARNESS_SCRIPT is not set"; fi
+
+# support multiple, space delimited, config files
+config_cmds=""
+for cfg in $MOZHARNESS_CONFIG; do
+  config_cmds="${config_cmds} --config-file ${SRC_ROOT}/${cfg}"
+done
+
+python2.7 $SRC_ROOT/${MOZHARNESS_SCRIPT} ${config_cmds} "${@}"
+
+
+

--- a/testing/taskcluster/tasks/branches/base_jobs.yml
+++ b/testing/taskcluster/tasks/branches/base_jobs.yml
@@ -311,8 +311,14 @@ tasks:
         - 'mobile/android/**/*.xml' # Manifest & android resources
         - 'mobile/android/**/build.gradle'
   mozharness:
     task: tasks/tests/mozharness-gecko.yml
     root: true
     when:
       file_patterns:
         - 'testing/mozharness/**'
+  marionette-harness:
+    task: tasks/tests/harness_marionette.yml
+    root: true
+    when:
+        file_patterns:
+          - 'testing/marionette/harness/**'

new file mode 100644
--- /dev/null
+++ b/testing/taskcluster/tasks/harness_test.yml
@@ -0,0 +1,70 @@
+# This is the "base" task which contains the common values all harness
+# unit tests must provide.
+# It's inspired by build.yml, since harness unit tests just depend on
+# checking out the source tree.
+---
+taskId: {{build_slugid}}
+
+task:
+  created: '{{now}}'
+  deadline: '{{#from_now}}24 hours{{/from_now}}'
+  metadata:
+    source: '{{source}}'
+    owner: mozilla-taskcluster-maintenance@mozilla.com
+  tags:
+    createdForUser: {{owner}}
+  workerType: desktop-test-xlarge
+  provisionerId: aws-provisioner-v1
+  schedulerId: task-graph-scheduler
+  routes:
+    - 'index.gecko.v1.{{project}}.revision.linux.{{head_rev}}.{{build_name}}'
+    - 'index.gecko.v1.{{project}}.latest.linux.{{build_name}}'
+  scopes:
+    - 'docker-worker:cache:level-{{level}}-{{project}}-tc-vcs'
+    - 'docker-worker:cache:level-{{level}}-{{project}}-test-workspace'
+  payload:
+    # 30 minutes should be more than enough for harness unit tests
+    maxRunTime: 1800
+    image:
+      type: 'task-image'
+      path: 'public/image.tar'
+      taskId: '{{#task_id_for_image}}desktop-build{{/task_id_for_image}}'
+    cache:
+      # The taskcluster-vcs tooling stores the large clone caches in this
+      # directory and will reuse them for new requests this saves about 20s~
+      # and is the most generic cache possible.
+      level-{{level}}-{{project}}-tc-vcs: '/home/worker/.tc-vcs'
+      level-{{level}}-{{project}}-test-workspace: '/home/worker/workspace'
+    artifacts:
+      'public/logs/':
+        type: directory
+        path: '/home/worker/workspace/build/upload/logs/'
+        expires: '{{#from_now}}1 year{{/from_now}}'
+    command:
+      - bash
+      - /home/worker/bin/build.sh
+    env:
+      # Common environment variables for checking out gecko
+      GECKO_BASE_REPOSITORY: '{{base_repository}}'
+      GECKO_HEAD_REPOSITORY: '{{head_repository}}'
+      GECKO_HEAD_REV: '{{head_rev}}'
+      GECKO_HEAD_REF: '{{head_ref}}'
+      TOOLTOOL_REPO: 'https://git.mozilla.org/build/tooltool.git'
+      TOOLTOOL_REV: 'master'
+      TOOLS_DISABLE: true
+      MOZ_BUILD_DATE: '{{pushdate}}'
+      MOZ_SCM_LEVEL: '{{level}}'
+
+  extra:
+    # These definitions are expected of build tasks but not used in our case
+    build_product: '{{build_product}}'
+    build_name: '{{build_name}}'
+    build_type: '{{build_type}}'
+    locations:
+        build: null
+        tests: null
+    index:
+      rank: {{pushlog_id}}
+    treeherder:
+      groupSymbol: tc
+      groupName: Submitted by taskcluster

new file mode 100644
--- /dev/null
+++ b/testing/taskcluster/tasks/tests/harness_marionette.yml
@@ -0,0 +1,37 @@
+---
+$inherits:
+    from: 'tasks/harness_test.yml'
+    variables:
+      build_product: 'harness_test'
+      build_name: 'marionette-harness-pytest'
+      build_type: 'opt'
+
+task:
+  metadata:
+    name: '[TC] - Marionette harness unit test'
+    description: 'Marionette harness unit test'
+
+  payload:
+    command:
+      - --tests=testing/marionette/harness/marionette/tests/harness_unit
+      # Avoid overwriting /home/workspace/build/src
+      - --work-dir=mozharness_workspace
+    env:
+      JOB_SCRIPT: 'testing/taskcluster/scripts/tester/harness-test-linux.sh'
+      MOZHARNESS_SCRIPT: >
+          testing/mozharness/scripts/marionette_harness_tests.py
+    artifacts:
+      'public/logs/':
+        type: directory
+        path: '/home/worker/workspace/mozharness_workspace/upload/logs/'
+        expires: '{{#from_now}}1 year{{/from_now}}'
+  extra:
+    treeherder:
+      symbol: Mn-h
+      tier: 2
+      machine:
+        # see https://github.com/mozilla/treeherder/blob/master/ui/js/values.js
+        platform: linux64
+    treeherderEnv:
+        - production
+        - staging