The 21-year-old, who is Leicester's first signing of the close season, had a successful 2018-19 campaign at Luton, playing 52 matches in all competitions and helping them seal promotion to the Championship (second tier) after a 12-year absence.
British media reports said Leicester had paid eight million pounds to Luton for the right back, who can also play on the left side of defence.
"I'm over the moon. I feel like I'm ready for this moment. I hope to bring a lot of energy and commitment to the badge," Justin told Leicester's website https://www.lcfc.com/news/1257449/james-justin-signs-for-leicester-city-on-a-fiveyear-deal/press-release.
Justin, who came through Luton's youth ranks, made his debut three years ago. He was called up to England's under-20 squad in Aug. 2017 and made his debut against the Netherlands shortly after.
He will link up with his new team mates on Monday as Brendan Rodgers' side begin preparations for the new season.
"The manager is great with younger players and developing them through their careers. Everyone speaks very highly of him and I hope he can help me too," Justin added.
Leicester, who finished ninth in the league last season, begin their new campaign against Wolverhampton Wanderers on August 11.
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by Toby Davis)