The 27-year-old has joined for an undisclosed fee, subject to league ratification, and has signed a three-year contract.
Cardiff were relegated from the Premier League last season and manager Ole Gunnar Solskjaer is now re-shaping the squad ahead of a Championship campaign.
Solskjaer has now made four signings this summer, Javi Guerra, Guido Burgstaller and Federico Macheda having already arrived.
Le Fondre said: "It's such a great opportunity for me to come here to Cardiff City and play for a boyhood hero of mine in Ole Gunnar Solskjær. It was a move I just couldn't say no to.
"I'm delighted to be working with Ole and I can't wait to get started.
"Scoring is what I'm about. I've often got into double figures during my career although, looking back at last season, it's one I really wanted to score more in personally.
"They are the high standards I set myself and now the aim is to do that with Cardiff City."
The Cardiff website added: "Le Fondre arrives with an impressive scoring pedigree, proven at every level of British football.
"In three years at the Medejski Stadium he netted 41 goals in 109 games, including 12 goals in the Barclays Premier League in 2012-13.
"All at Cardiff City FC welcome Adam's arrival and thank our colleagues at Reading for their assistance in the transfer."
Reading's director of football Nick Hammond said: "I'd like to thank 'Alfie' for everything that he has done for Reading Football Club. His application and attitude have always been first class.
"We are all disappointed to see him go but, in these challenging times, this is a vital and necessary deal for the club, and a deal that makes financial sense for both the club and the player. We wish Alfie and his family all the best."