The 22-year-old is currently in New Zealand with the England squad, hoping to add to his 17 senior caps, and will join Tigers for pre-season training after a summer break.
"Leicester is a world-class club and the ambition of the club is very exciting for me," said Flood, who made his Premiership debut for Newcastle Falcons as a 19-year-old.
"I am sad to leave home and to leave Newcastle where I have very fond memories, but at this stage of my rugby career a new challenge is very important.
"This is an exciting challenge and it is one that I could not turn down. I am looking forward to getting stuck in at Leicester in pre-season training."
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