The signing has come out of the blue with Ainsworth a regular in Wimbledon side. His last appearance for the Dons was only 4 days ago in a 3 - 3 draw at Gillingham and only 11 days ago he scored against league leaders Portsmouth in a 2 - 1 Wimbledon win.
The new broke in the Echo today with the following story
He will train with the Bluebirds tomorrow and will be plunged straight in to the Bluebirds squad for Tuesday night's Nationwide League home match against Blackpool at Ninian Park (7.45pm).
"He's fast, direct and can score goals," said Cardiff City manager Lennie Lawrence. "Gareth can unlock defences."
Ainsworth, 29, has been playing for Wimbledon in Division One this season and it was Cardiff City owner Sam Hammam who brought him to the London club in November 1998.
The Dons paid £2m for Ainsworth, who started his professional career 11 years ago when Preston North End plucked him from non-League football with Northwich Vics.
He has joined Cardiff for a "small fee" and they were able to persuade Wimbledon to part with the talented wide player because he is out of contract at the end of this season.
"I took him to Wimbledon and I know him well," said Hammam. "I'm delighted Lennie has decided he can help our cause.
"Gareth is like a tank. He take possession and dribble past defenders. He can score goals and put in fantastic crosses.
"He is a player who can unlock defences."
Blackburn-born Ainsworth was with his home town club Rovers as a youngster. He has played for Preston in three separate spells, Cambridge United, Lincoln City, Port Vale and Wimbledon. He has found the net more than 80 times in almost 350 first-team appearances and played for Wimbledon in the Premiership.
During his two-year spell at Lincoln City, starting in 1995, Ainsworth scored 37 goals in 83 League appearances.
But his stay with Wimbledon has been plagued by injuries, although he has been fully fit this season, and Hammam said: "He had never been injured in his life, but he had to have a groin operation after scoring five goals in his first two matches for Wimbledon. He seemed to go from operation to operation for a long time, but he has come through that now."