Isaacs, capped at Wales Under-20 and Sevens levels, has put pen to paper on a deal at BT Sport Cardiff Arms Park until at least 2016.
The 27-year-old previously played at Newport Gwent Dragons before a successful five-year spell at the Ospreys where he made a total of 66 appearances.
The Cardiff-born back, an IRB Sevens World Cup winner with Wales in 2009, started his career as a scrum half before being converted to centre.
Director of Rugby Mark Hammett said: "Tom strengthens our squad and will add another dynamic to our midfield options. We've been hit hard in the centre, with injuries and international call-ups, and so Tom's signing is a very important one.
"He is a strong ball carrying centre who can make things happen and I've been impressed by his tremendous attitude towards both training and games.
"We're excited to welcome him back to his home city and look forward to seeing him in action for Cardiff Blues."
Isaacs said: "I'm delighted to be joining Cardiff Blues and excited about my future at the region and taking this opportunity.
"I'd like to thank everyone at Gloucester Rugby for their support during my time in England. Gloucester is a great traditional rugby club but, with a change in rugby management after my signing, opportunities have been limited this season.
"I want to be playing regular rugby and couldn't turn down the chance to return to my home city. I played all my rugby growing up for Ysgol Glantaf and Llandaff North before time with Cardiff RFC.
"I'm excited to now be returning to BT Sport Cardiff Arms Park and being part of the future at Cardiff Blues."
Factfile:
Full name Tom Isaacs
Born February 18, 1987, Cardiff
Major teams Newport Gwent Dragons, Ospreys, Gloucester Rugby, Wales Sevens
Position Centre
Height 6ft 0in
Weight 93kg
Guinness PRO12
Suite 208, Alexandra House,
The Sweepstakes
Ballsbridge, Dublin 4, Ireland