The Brazilian winger – who enjoyed a successful spell on loan at St. James' Park last term – returns to the club until the end of the 2018/19 season.
Kenedy is well-known to United supporters, having spent half of the 2017/18 season on Tyneside as Newcastle secured a top-ten finish under manager Rafa Benítez.
He has joined up with the rest of the Magpies squad at the club's pre-season camp in Ireland and will train this morning.
"I am so happy with this interest from Rafa Benítez, that he came back for me," said Kenedy. “I want to keep it going, the work that I did here before, for the new season.
"I have received many, many messages on social media from the fans. They made me feel appreciated, so I wanted to come here in this new season and give everything on the pitch."
Manager Benítez added: "I'm really pleased to have Kenedy back with us. I'm sure every fan will be happy with this signing.
"Hopefully he can keep playing at the same level that he reached with us last season. I wish him all the best for his time here."