The 27-year old Welsh international centre has committed to his home region until the end of the 2015/16 season.
Capped by Wales at age-grade level, Spratt came through the development pathway at the Ospreys, via Neath Athletic and then the Welsh Premiership with Neath, before securing his first regional contract in 2007 - after making his debut as a second half replacement against Munster in Thomond Park during the 06/07 season.
Having impressed during a loan spell with Air New Zealand Cup outfit Taranaki in the summer of 2008, Spratt gained the first of his four Wales caps the following year, against Canada in Toronto. He spent 2011/12 with London Irish in the Aviva Premiership, playing 11 times for the Exiles, and has established himself as a key figure in the Ospreys squad since returning to the Liberty Stadium two years ago.
Now with 54 appearances for the Ospreys to his name, scoring 10 tries, he has led the team as captain during both international windows this season.
After putting pen to paper on his new deal, Spratt spoke about the positive future that lies ahead for the Ospreys.
"It was an easy choice for me" said Spratt.
"This is my home region and the last two year's in particular have been really good for me personally. These are really exciting times for the Ospreys moving forward and I'm really keen to be a part of it.
"We've got a nice blend of experience and some really exciting, youthful players coming through. The region has re-signed some big players which is a positive for everyone involved with the Ospreys and is a sign of the intent we have here. With good players and coaches working in this environment, we've got a bright future ahead. I'm just delighted that I'm going to be involved for another two years."
Andy Lloyd, Rugby Operations Manager, paid tribute to Spratt's professionalism, stressing his importance to the Ospreys on and off the field.
"We are absolutely delighted that Jonathan will remain as an Osprey for the next two years" Lloyd said.
"He has been an invaluable squad member of the past two seasons since we were able to bring him back from London Irish, and is a consummate professional whose maturity and support has been and will continue to be crucial as this group continues to evolve, typified by his current standing as captain.
"Equally as importantly is that his own game continues to flourish, with his creative edge and smart running lines to the fore, as we strive to develop an all round game that can be both dynamic and successful. It was great to see him getting national recognition last summer and with strong competition for places ahead I know that he will respond to the challenge and keep driving his own and others high standards, something that is crucial for the ongoing development of the team.
"He is someone who has always been recognised as having the potential to be a real positive influence here at the Ospreys, and is now fulfilling that, using the experience he has picked up in different environments to really benefit himself and the group.
"Along with recent announcements about the re-signing of Alun Wyn Jones, Ashley Beck and Eli Walker, this is another positive statement about our future."
The Ospreys have also confirmed that full-back Dan Evans and hooker Sam Parry will join the region from Newport Gwent Dragons in the summer, along with Fijian international centre, Josh Matavesi, who joins from Worcester Warriors.
Guinness PRO12
Suite 208, Alexandra House,
The Sweepstakes
Ballsbridge, Dublin 4, Ireland