Roseman will leave Paul Robinson, with whom he has operated the new writing venue since November 2006, as sole artistic director.
According to a statement released by the venue, Roseman departs Theatre503 to return to his freelance career and upcoming projects in Sydney and the West End.
Since taking the reigns in 2006 Roseman and Robinson have premiered 48 new plays and staged work by 140 first time writers.
In 2010 the theatre's production of David Harewood's The Mountaintop transferred to Trafalgar Studios, winning an Olivier Award for Best New Play. Their work has also been nominated for a further 17 major awards, including a further Olivier, two Evening Standard and two Whatsonstage.com Awards.
Speaking about his departure Roseman said in a statement: "I am honoured to have played a small part in making Theatre503 a beacon for new writing. I have been privileged to lead this organisation with my dear friend Paul, and am delighted that he will take the helm to reinvent and re-imagine the theatre moving forward."
Paul Robinson said: “I shall miss Tim hugely but wish him so much love and luck for the future. I gain immense strength from the artistic vision, energy and commitment Tim has injected into the theatre and the work we have created together.
"Now that we have worked so hard to establish 503 I’m thrilled by a raft of new possibilities and will relish making the theatre bolder and braver than ever. I’m really excited about a new era and can promise that 503 will continue to be a vital destination for the most exhilarating new writing."
Currently playing at Theatre503 is Roseman's production of Craig Higginson’s The Girl in the Yellow Dress, which comes to the theatre following a 2011 run at Salisbury Playhouse. The production continues until 14 April.