Thomas, who has just agreed terms on a new one-year deal with Cherries, will return to the Conference South outfit on a season-long loan.
The 19-year-old stopper had two spells with the Magpies last season before being recalled on both occasions due to disciplinary issues surrounding fellow keeper Jon Stewart.
Thomas will continue to train daily with Cherries goalkeeping coach Neil Moss and will feature in the club's pre-season campaign before joining Dorchester.
A product of the club's youth system, Poole-born Thomas made four appearances during Cherries' League Two promotion campaign but only featured on the bench this season.
Cherries boss Lee Bradbury, who will have a recall clause in Thomas's agreement, told the Daily Echo: It will work well for Dan and the two clubs. He will train with us and we can continue to improve him.
We can also keep an eye on his progress and, at the same time, he will play a decent standard of football.
I think he will progress a lot quicker doing that than he would playing 15 reserve games for us.
Bradbury confirmed he was still hoping to sign Billy Lumley from the Glenn Hoddle Academy to provide back up for Cherries number one Shwan Jalal.