The 26-year-old quit the Exiles just one year after moving to the Madejski Stadium and has penned a two-year deal with the Welsh region.
"I'm very excited to be back here. It's where I'm from, I was brought up playing rugby in Ospreylia, so clearly when the opportunity arose to come back I jumped at the chance," said Spratt, who came off the bench for Wales against Canada in 2009.
"Having spent a year at London Irish and gained some great experience, the changes there meant I had to consider my options and while I could have stayed, the prospect of rejoining the Ospreys and being part of what is a young and ambitious group is an exciting one.
"There's nothing like playing for your home region in front of a big crowd at your home stadium.
"Working with Steve Tandy, Gruff Rees and Jonathan Humphreys is another big draw for me. Having played alongside Steve at Neath and the Ospreys, I know what he's about, what his work ethic is, his passion for rugby in general and for the Ospreys in particular, and I'm really looking forward to the new season.
"I was fortunate enough to be able to have the experience of playing in the Aviva Premiership and the Heineken Cup at London Irish last season and I'm a better player for those experiences than I was 12 months ago.
"All any rugby player wants is to be able to cement a regular team place and that's something I'm going to be striving towards over the next two years, to hopefully find myself in the Ospreys first fifteen. Competition is tough here but it's up to me to prove myself."
Spratt had two years left on his contract at the Madejski Stadium but was allowed to leave following the departure of Toby Booth.
He also spent time away from the Liberty Stadium when he went to Tarankaki, New Zealand, on loan in 2008.
And Ospreys head coach Tandy is confident Spratt has learned a great deal from his stints in different rugby environments.
"It's good to be able to bring Jonathan back to his home region," added Tandy.
"He's passionate about this place, loves the Ospreys and will be a real asset this season. He's a skilful player who has always been underrated, with a good attitude, someone I know well from my time as a player in the same squad.
"He has gone out there and gained different experiences, playing in New Zealand as a youngster and then going to London Irish and showing that he can compete at Aviva and Heineken level, and he is going to be a valuable addition to the group."
Spratt is the second player to return from London Irish this summer, following Tom Smith's U-turn last month without playing a single game for the Ospreys.