"@martunisCR8 has joined the Club! The young boy that survived the tsunami will work and grow at #SportingCP," Sporting said on its official Twitter feed.
Martunis was found alive 21 days after Indian Ocean tsunami hit Aceh province on December 26, 2004.
His mother and two siblings were killed.
He was wearing a Portugal national team jersey when found in a swampy area after drifting for days at sea.
His story moved Ronaldo, who visited him in Aceh in 2005 and bought him a house.
Martunis has Christiano Ronaldo's initials on his Twitter feed, which also shows him in a picture with his idol.
The news was greeted with enthusiasm in Indonesia, where football has come under the spotlight for more negative reasons in recent weeks.
The national football association is under FIFA suspension over government interference.
"Martunis sends shock waves," read a headline on Indonesian football website soccer.co.id.
"At the time when Indonesian football is facing uncertainty, good news emerged from a faraway land, Portugal," it said.
"Bravo Martunis. What an inspiring story," said Annisa Pohan, a daughter-in-law of former president Susilo Bambang Yudhoyono, on Twitter.