Insua, who spent last season on loan in Turkey with Galatasaray, has signed a five-year contract with Sporting.
Insua has previously been linked with a move to Italian side Roma.
“I am delighted to start training and playing here,” he told Sporting’s official website.
“Sporting is one of the big Portuguese clubs and is recognised for having top players.
“The club is always looking to win titles. I will give everything for Sporting.”