"I want to thank both Richmond and Port Adelaide for giving me the opportunity to play AFL football, especially Richmond for giving me a second chance," Thomas said.
"Now is the right time to retire. I feel I have got everything I can out of my ability and have no regrets.
"I have thoroughly enjoyed my time at Richmond and thank you to all the Tiger fans for their support.
"I sincerely hope the club enjoys the success it deserves in the future."