The Yeovil-born striker signs an 18-month contract at Huish Park to become a permanent Glover until at least June 2020 subject to EFL approval.
Spending the second half of the 2017/18 campaign in Somerset, Seager made seven appearances scoring once before returning to his parent club.
Fast-tracked through the youth ranks at Saints, the 22-year-old was prolific throughout the age groups to sign a first professional deal in 2013.
Two years later, and fresh from scoring 16 goals for the Under-21s, Seager made his first-team debut by coming off the bench in an Emirates FA Cup tie against Crystal Palace.
Developing by gaining experience away from St Mary’s, Ryan spent time out on loan with Crewe Alexandra and MK Dons before arriving at Huish Park for the first time last January.
Having returned to Hampshire in the summer he was sent on loan to SC Telstar in the Dutch second division and now heads home on a permanent basis.
Speaking to YTFC.net, manager Darren Way said: “The opportunity to bring a player born and bred in Yeovil to the club on a permanent basis, couldn’t be missed.
“We know Ryan from his time with us last season but from seeing him in training we’re dealing with a different mindset this time around.
“I’m excited to be working with Seags once again and look forward to seeing what lies ahead.”