Cardiff-born Delve won 11 caps while playing for Bath and Gloucester between 2006 and 2010 before leaving for a Super Rugby deal with Melbourne Rebels.
"I'm really excited about this move," Delve, 32, told the region's website.
"It's a great opportunity for me to play with a quality team that I believe is going places."
Delve captained Melbourne, including a match against the 2013 Lions, while he also made a Wales tour appearance in a non-Test match in Australia in 2012.
He has been playing in Japan with the NEC Green Rockets for the past two seasons.
Delve's signing has been prompted by Ospreys having up to five back-row players involved in the World Cup next season.
"I've been really impressed by [Ospreys coaches] Steve Tandy and Andy Lloyd when we've met and I'm looking forward to helping them to achieve the vision that the Ospreys have," Delve added.
"There's a strong, competitive squad of players at the Ospreys, with some great young players who have real potential alongside some genuine world-class individuals.
"When you add some of the signings confirmed for next season, experienced people like Paul James and Brendon Leonard coming in, and Joe Bearman re-signing, I think it's a great blend."
Ospreys rugby general manager Andrew Millward told BBC Wales: "He'll [Delve] be challenging for a place week in, week out.
"We've got a lot of internationals in the back-row and he'll be adding to that mix, especially when some of the guys are away on international duty.
"What he brings is leadership, and what he can impart to the younger players is just as important."
"I've got no doubt he's kept himself in good shape in Japan. It's great that we've got such good representation [in the World Cup], we've just got to deal with it."
Delve can play blindside or number eight but will be competing with the likes of Dan Lydiate, James King, Dan Baker and Canadian Tyler Ardron for those positions.
Back-row Gareth Delve has signed a two-year contract with Ospreys and will now play professional rugby in Wales for the first time in his career.