The arrival of the duo concludes a busy week for Killie boss Allan Johnston as he continues to restructure the Rugby Park squad.
"Kilmarnock Football Club is pleased to confirm that we have signed Barry Nicholson & Mark Stewart until January 2014," a statement from Kilmarnock read.
"We would like to wish both players every success during their time at the club."
Nicholson made his name at Dunfermline before the midfielder spent three years at Aberdeen prior to a switch to Preston.
Forward Stewart started at Falkirk, and after a spell in England moved to Dundee, where he played 17 games last season in the top flight.
Full-back Sean Clohessy and midfielder Jackson Irvine were both signed earlier this week and are included in the squad to face St Mirren today.