Ultimately he scored 57 tries in 141 appearances for the Welsh side, including a fairytale double in his final-ever match for the region - the 2012 PRO12 Grand Final win over Leinster in Dublin.
And after receiving his award at a ceremony on Friday night, the 38-year-old insists he is still coming to terms with it.
"I'm shocked to be honest, it's not something I anticipated," Williams told Ospreys' official website.
"There are so many other people who have been involved over the last 12 years who could have been worthy winners of this award so it's pretty humbling."
During his international career, Williams scored 58 tries for Wales and a further two in his four Tests for the Lions as well as picking up the IRB World Player of the Year in 2008.
Be part of the biggest Tour in the world and register your interest in travelling with the British & Irish Lions to New Zealand in 2017