The 17-year-old signed for United in the final minutes ahead of the closing of the transfer window for around the £400,000 region.
Upon the agreement of his transfer fee, Poole accepted personal terms with the club before completing his medical in order to finalize the switch.
The centre-back is now set to feature for the club’s academy and has already represented the Wales Under-17 team.
He also became Newport’s youngest-ever player when he made his debut for the first team team earlier last year, with manager Terry Butcher revealing that turning the Red Devils down was impossible for the player.
“In the past few days a number of clubs have shown an interest in Regan, but as soon as Manchester United came in there was only really going to be one outcome,” he said.
“It is every boy’s dream to play for Manchester United, and it’s a wonderful opportunity for him to progress at such a magnificent club.
“It doesn’t matter which country you are in, or where you are in the world, when Manchester United come calling you have to answer.”
Poole played a total of 12 times in all competitions for his now former League Two side last term, but is not likely to feature for the senior United team for at least a few more years.