Lewis, who made 29 National League appearances on loan at Woking last season, played under Crawley boss Harry Kewell when the Australian was coach of Watford's under-23 team.
“I'm excited to get started now," he told Crawley's website.
“It's a new challenge but one I'm looking forward to.
“We've had some tough sessions and the Head Coach is working the boys hard.
“It's really important that all of us are fit for the season ahead and that's the priority at the moment."