The left-back signed a five-year contract on Tyneside. Lewis reportedly drew interest from Premier League champions Liverpool earlier this summer but instead joined Newcastle for £13.5 million, according to Mark Douglas of the Newcastle Chronicle.
"Jamal is a very exciting prospect. I watched him flourish in the Championship two years ago and he made a real fist of it in the Premier League last season," said manager Steve Bruce.
"He is only 22 years old and has a wonderful future ahead of him so I hope he is a Newcastle United full-back for years to come."