Guinan, 34, scored in the FA Cup against Rovers this month, and for Cheltenham in the 2006 play-off final.
Rovers manager Dave Hockaday told BBC Gloucestershire: "Steve lives in Worcester and we were aware of his availability earlier in the season.
"He played well against us and we had a few conversations. Other clubs were in for him but he's a great capture."
Rovers will the 13th different club Guinan has played for in a career which has yielded 106 goals in 432 matches.
Hockaday added: "I think Steve will be great for Reece (Styche) and Ross (Dyer) because he almost forces you to pass the ball, spin off each other and work off each other.
"He's got good movement, good knowledge and I know he will help my two young striker become better players. The idea is we'd like to make it more permanent but at the moment it's a loan until January."
Source: BBC Sport
Source: BBC Sport