The 20-year-old attacking midfielder has agreed a a two-year deal at the County Ground with a one-year option.
Stewart has spent his entire football career to date in his native Northern Ireland and has earned international honours with his country’s Under-19s. He recently received a call-up to the Northern Ireland Under-21 side.
The youngster is still awaiting international clearance so is ineligible to play for Town this weekend at Southend.