Spratt made his Ospreys debut aged 20 and had a loan spell with New Zealand side Taranaki in 2008.
A year later he made his Wales debut against Canada before spending 2011-12 at London Irish.
Law graduate Spratt made his 100th Ospreys appearance in a win against Wasps in the Anglo-Welsh Cup in February 2017.
Spratt said: "This is the right time for me to move on to the next stage of my life.
"I've had a good career. I was only 20-years old when I made my debut and I realise that most professional rugby players don't get as long in the shirt as I've had so I'm extremely grateful to have been given the opportunity and to have been able to do the dream job as long as I have."
Ospreys' four-times capped Wales centre Jonathan Spratt is to retire at the end of the 2016-17 season at the age of 31.