Scarlets chief executive Stuart Gallacher said: "We thank Sililo for his contribution and we wish him all the best for the future."
New Zealand-born Martens joined the Scarlets from Sale Sharks and made his debut last August against Bath.
His final run-out in a Scarlets shirt came on January 31 in the Barbarians game which marked the official opening of new stadium Parc y Scarlets. He came on as a second-half replacement.
Martens' international debut for Tonga came in 1998 when he played Australia and Fiji. He has played a total of 42 times for Tonga and three times for the Pacific Islanders - last year against England, France and Italy.
Martens said: "I wish the Scarlets all the best for the coming years. With the squad they have and their new stadium they have a big chance to build a great future.
"I thank the Region for all they've done for me and thank my teammates and fans for all their goodwill and support.
"I'm now talking to a couple of clubs about playing contracts for next season and will also be able to spend important time
on pursuing my own business ventures."
One of these is Pacific Nations Sports (www.pacificnationsrugby.com), a support base for Pacific Islands players contracted to European teams. They are due to host their first charity awards dinner at Twickenham on April 28.