Johan Sebastian Ljung told the Phuket Provincial Court that he was shocked by the sentence because he had pleaded guilty.
He said he could not accept the sentence was so severe because he had no dispute with the murdered man, Maksim Schantz. Ljung has 30 days in which to appeal the sentence.
Another Swede, Tommy Viktor Soderlund, pleaded not guilty to murder and has been tried separately. He is likely to be sentenced on March 3.
Schantz's mother Yvonne sued the two men over the murder and will receive the 12.5 million baht.
Both Swedes insisted that murder was never their intention. They had come to Phuket from Pattaya to talk to Schantz, who allegedly stole a list of potential Swedish victims of a call centre scam being operated by Soderlund from Pattaya.
Ljung accompanied Soderlund to Phuket but did not previously know Schantz. On the night of August 1, 2011, the three men met and scuffled outside Shantz's rented three-storey duplex in Phuket City.
When Schantz, who was wearing a full-face motorcycle helmet, began to struggle, Ljung grabbed him by the shirt collar and the knife severed an artery in Schantz's neck.
As Schantz bled to death, the two men allegedly fled through a back fence, ordering a local resident at gunpoint to surrender his motorcycle. The pair were arrested the next day in Cherng Talay, in Phuket's north.
As well as life in jail for murder, Ljung was also sentenced to two years in jail for possession of a handgun in a public place and six months for carrying a knife.
.Schantz's mother said outside the court today that she was satisfied with the verdict and sentence.
She had been living in Pattaya with her son but returned to Sweden with his ashes after his cremation.
She carries a photograph of her son in her handbag and is staying in Thailand until after Tommy Viktor Soderlund is sentenced.