Belgian defender De Laet, 23, and English midfielder James, 20, go to the King Power Stadium on three year contracts as Leicester look to strengthen for next season.
De Laet moved to United from Stoke City in 2009, he has made six appearances in the first team but he has been out on-loan to clubs like Sheffield United, Preston, Portsmouth and Norwich.
James is a former England Under-19 captain, but he never managed to break into the United first team, the season just gone saw him suffering with injury although he did have two spells on loan with Preston before.
Speaking to the club’s official website, manager Nigel Pearson said: "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."