The 28-year-old started his league career at Nottingham Forest but joined Hereford two years ago.
Guinan played three time for United after joining on loan in September 1999, scoring once.
He was famously ordered to hand back his club tracksuit in the car park by manager Brian Laws after saying he didn't want to stay with the club.
Cheltenham Town manager John Ward thinks he has signed a player who will socre plenty of goals.
He said: "Steve has an impeccable goal-scoring record.
"He has averaged one every two games for the past two seasons for a good club in a good league."
More players are expected to follow Guinan to Whaddon Road after nine players were released by Ward.