The former Manchester United winger is a free agent after leaving Aston Villa during the summer and has been training with the Bluebirds.
Richardson becomes new boss Neil Warnock's fourth signing, after defender Sol Bamba, winger Junior Hoilett and striker Marouane Chamakh.
"I'm honoured to play for a big club like Cardiff City," Richardson said.
"I can't wait to get started now.
"I spoke to the gaffer who said the right things. I knew straight away I wanted to play for him.
"I looked at a few options after leaving Villa, but everything here fitted the bill."
Manager Warnock added: "Kieran is another experienced player who has committed to us until January - and as I see it that is a no lose situation.
"He's a quality player, established at top level and can contribute towards us moving up the table."
Ex-Arsenal and Crystal Palace striker Chamakh has also only signed a short term deal until January and Warnock says that he must prove himself if he wants to extend his stay.
"He has a bit of a reputation in certain areas," Warnock added.
"One or two people in the club mentioned to me about his reputation and they mentioned three of four things that he's renowned for. I added a couple to that, hence why we've taken him until January."
"I think he's changed as a person. His partner is here with him and he's got a little baby now. And I think that will be good for him."
The Moroccan international was released by Crystal Palace in July after failing to score in 12 games last season and had an unsuccessful trial this summer with West Brom.
"It's not a long term commitment from the club. It gives him an opportunity to show what he can do" Warnock continued.
"He can play up front or in the hole behind. He's got a good football brain. So it's a matter of if he enjoys it he'll stay. If he doesn't do a job he'll leave.
"He's had a few offers over the past few days from abroad. But he wants to stay in England."
Richardson could make his debut when Cardiff host Bristol City on Friday night.
England international Kieran Richardson has joined Cardiff City on a short-term deal until January.