Premiering at the Berlin Film Festival, Unsane tells the story of Sawyer Valentini (played by Claire Foy), who moves to a new city to escape her stalker David but finds herself admitted to a mental health institution where he works.
Sawyer is convinced she has been wrongly admitted to the facility but no one believes her so she is trapped there and subjected to torments from David.
Soderbergh said the overall experience of making a film in just two weeks on an iPhone was good, although there were some drawbacks such as the phone being very sensitive to vibrations.
"I have to say the positives for me really were significant and it's going to be tricky to go back to a more conventional way of shooting," he said in Berlin.
Not having to make a hole in a wall or secure a camera to the ceiling are big advantages, as is being able to go straight from watching a rehearsal to shooting, Soderbergh said.
"The gap now between the idea and the execution of the idea is just shrinking and this means you get to try out more ideas so I wish I'd had this equipment when I was 15," he said.
Joshua Leonard, who plays David, said filming on an iPhone enabled the actors to stay in the world of their characters and the film more than the conventional camera set-up would allow.
"There's nothing more fun as an actor than just being in the thick of the creative process when you're actually on set and not having to wait for the machine of filmmaking to catch up with the creative impulse," he said.
Being used to people putting iPhones close to his face to take selfies helped too because "it really minimised any self-consciousness about the process of making a film", he said.
Unsane opens in Australia late April.