The Samoan powerhouse has signed a one-year deal just days after Blues coach Dai Young revealed key-man Harris would be returning to his rugby league roots.
But Young is confident 33-year-old Tuilagi has what it takes to help fill the void in time for the coming season.
"It's great news for us," said Young, who is currently in New Zealand working alongside All Blacks and former Wales coach Graham Henry ahead of Saturday's test with England. "We have been looking for a player who can step up and make an impact on the game both offensively and defensively.
"His direct style of play and blistering pace will add a whole new dimension to our back line."
Tuilagi leaves Welford Road after four seasons with the English giants, the pinnacle of his move from the 13-man code coming with a Heineken Cup win over Minster back in 2002.
And the Samoan international, who can operate at either wing or centre, has been backed by Blues chief executive Robert Norster to make an immediate impact in his new surroundings.
"Freddie is a player of vast experience given his achievements at both club and international level," he said. "Not only that but he is also an experienced rugby league professional and a clinical try scorer in both codes with a dynamic style that has delighted supporters throughout the rugby world."
Tuilagi is the Welsh region's ninth signing of the summer following the likes of Gethin Jenkins, Robert Sidoli and Deniol Jones from the defunct Celtic Warriors.