Belgian defender De Laet, 23, and English midfielder James, 20, will move to the King Power Stadium on three-year contracts, the Championship club announced.
De Laet (pictured) had a loan spell with Norwich last season which was terminated early in January, after previous short-term stints at Sheffield United, Preston and Portsmouth.
He made just six first-team appearances for United after a surprise move from Stoke in 2009.
Former England U19 captain James did not make a first-team breakthrough at Old Trafford. He spent two loan spells at Preston in 2010 but was troubled by injury in the season just ended.
Leicester manager Nigel Pearson told his club's website: "Ritchie and Matthew are both players we have been monitoring for some time and we are very pleased to be able to bring them both to the club.
"Both are hungry, young players with winning mentalities, experience of Championship football and a desire to be successful."