Theroux, 44, shared with fans a black and white selfie, which he simply captioned, "#on", reported People magazine.
Many of his new followers expressed how happy they were with his decision to join Instagram, and even asked for photos with his new wife, Jennifer Aniston, who has been open about not wanting to join social media.
While she hasn't jumped on the Instagram train herself, Aniston, 46, did take over Living Proof's page earlier this summer and said that it was not a pleasant experience.