Summerbee left Swindon in June 1994 at the end of Swindon's only Premiership season to join Manchester City in a £1.5m move.
He's also played for Sunderland, Nottingham Forest, Leicester City and more recently has been with Bradford City before being released.
He's now 33 years old, and has been given the squad number 23. He's joined Swindon on a non contract basis, and goes into the squad to face Nottingham Forest later today.