A320 First Officer reported an ECAM warning for the #1 engine's thrust reverser being unlocked appeared just after takeoff. The flight crew ran through checklists and then performed an air turnback.

Narrative

After takeoff passing through approximately 700 ft. AGL; the ECAM for #1 thrust reverser unlocked appeared. I was flying; announced my aircraft" and the Captain checked the card for Quick Action or ECAM exceptions. Seeing none; he assigned me flying duties and I called for ECAM actions. We also completed the After Takeoff Checklist at that time. We requested level off and worked the ECAM; which included shutting down the #1 Engine. During this process; we received radar vectors onto downwind. Once we had completed the ECAM actions; we entered the holding fix for the ILS XXR at ZZZ. The Captain notified Dispatch; the FA (Flight Attendant) and made an announcement to the passengers. In the hold; we completed the Overweight Landing Checklist and pages for abnormal landing. Once we felt we had completed all necessary actions; we accepted clearance for the ILS XXR and completed all applicable checklists. We taxied clear and were inspected for any possible damage to the #1 Engine and to check brake temperatures. Finding no issues; we coordinated with ZZZ Operations and returned to the gate. After landing; Maintenance found that the thrust reverser had slightly deployed."