After his West Ham contract was allowed to expire at the end of last season, Carroll is available on a free transfer.
Although Carroll is still recovering from ankle surgery in April he passed a medical and Steve Bruce hopes to be able to involve him in a first-team game before next month's global break.
Talk emerged of an unlikely return to Newcastle and a one-year deal was sealed on Thursday ahead of the transfer deadline. "He is very excited to be home", the manager, who moved to Newcastle from Sheffield Wednesday in July, said.
Carroll can be a useful alternative off the bench if he gets back to fitness, but his best days are long behind him.
His progression continued apace in the top flight with his 11 goals by Boxing Day, which included a hat-trick against Aston Villa, prompting Liverpool to swoop on the final day of the winter transfer window in January 2011 with an offer Newcastle simply could not refuse.
If that exit upset many Newcastle supporters, it did not provoke anything like the disappointment engendered by Rafael Benítez's resignation in June and his replacement by Bruce.
Carroll is the fifth player to join the club since Bruce's appointment, following in the footsteps of record capture Joelinton, Allan Saint-Maximin, Jetro Willems and Emil Krafth.
Newcastle open their Premier League campaign at home to Arsenal on Sunday (14:00 BST).
Carroll is expected to complete an emotional return to the club and will fight for a spot in the side.
If Carroll can keep himself fit, play a few games, and score 10 goals over the season, it would have been a very good decision by Bruce to sign him. The club said they're "delighted" to announce the return of Carroll. "There are times where I wonder if he knows what he's doing, but he's got that natural flair that gets you off your seat". It's been a pretty tough time since then.