Arihano Haiti was found guilty of murdering 40-year-old Stefan Ramin, whose charred remains were found days after he went missing on the island of Nuku Hiva during a round-the-world sailing trip with his girlfriend, Heike Dorsch, 37.
The 33-year-old was also convicted of sexually assaulting Dorsch, and ordered to serve 28 years in jail, with no possibility of parole for 18 years.
Arihano Haiti had surrendered to military police after seven weeks on the run.
He admitted shooting Ramin in the head, although his motive remained unclear throughout the trial.
Dorsch had told investigators Ramin headed for the interior of the island on a hunting trip with Haiti, who returned alone to the couple's boat hours later to tell her that her partner had been injured and needed help.
She claimed she followed the guide, who tried to assault her after tying her to a tree, but she managed to escape.
Haiti claimed in his defence that Ramin had plied him with drug-laced rum before attempting to rape him. But investigators found no evidence to support his claim.
Ramin's remains were found in a charred pit on the island.
His killing led to media speculation that he had been the victim of cannibalism, but the prosecution quashed the rumours.