Radwan joins a Leicester team that is fourth in the Premiership and through to the last 16 of the Champions Cup after finishing third in their pool.
He said he had “nothing but positive memories” of his time at Newcastle.
“The supporters and everybody at Kingston Park have always been great with me, and I can’t express how much that has meant,” he added.
Falcons consultant director of rugby Steve Diamond said he understood that some fans would be disappointed by Radwan’s departure, but expressed that reluctantly it was the right thing for all parties, with Newcastle receiving compensation from Leicester for the winger leaving in mid-season.
“When a player of Adam’s ability comes to see you and says: ‘Look, I want to take the opportunity that has come up’ then we have to be honest with ourselves,” he said.
“He’s a very good player but I don’t want to stand in people’s way. We’re in transition and a good opportunity has come up for him to go to Leicester.
“I’ve had a long conversation with him over this. It’s not happened overnight. Three or four clubs approached us and I think this is the right thing for him to do.
“I could have convinced him to stay to the end of the season, but if I was in his shoes, how would I want to be advised? I care about Newcastle massively but equally I have a duty of care to lads like Adam and the advice is the right thing.”
Leicester are next in action with an away game against Gloucester on Saturday.
