The 26-year-old returned to the Knights in 2016 after two seasons at the Dragons, but was unable to come to an agreement on a contract extension, ending his time in the Hunter for a second time.
Mata'utia never missed a game for the Knights this season and was regarded as one of their more consistent players on the paddock, scoring seven tries and averaging 85 metres.
He took to social media to deliver the news on Thursday.
