"Arthur," Phillips captioned the image of Phoenix, who stares at the camera -- unsmiling and without the titular villain's signature clown makeup.
Phoenix, 43, has unkempt, collar-length hair and is wearing a rumpled, button-down shirt and jacket in the picture.
Phoenix's previous films include To Die For, Gladiator, Quills, Her and Walk the Line. Phillips is known for helming The Hangover comedy trilogy.
Warner Bros. plans to release the Joker origin film on Oct. 4, 2019.
The dark-toned movie is to take place outside of the existing DC extended film universe and is intended to help launch a new DC Comics banner that will allow the movie studio to tell unique comic-book stories using different actors in the title roles.
The Joker was last portrayed by Jared Leto in Suicide Squad. He is expected to reprise the role in a Suicide Squad sequel and in a standalone film he is executive producing.