The high court upheld the acquittal of another suspect involved in the case.
Samson D'Souza and another man were accused of plying Scarlett with drugs, raping her and leaving her on the popular Anjuna beach, where she drowned.
Tests revealed she had ecstasy, cocaine and LSD in her system when she died.
Vikram Varma, a lawyer representing Scarlett’s mother Fiona MacKeown, said he was happy with the court’s decision.
It has taken a lot of time but justice has been done, he said.
Police originally said Scarlett had drowned after taking drugs but changed their story after her mother complained. D’Souza’s sentencing comes after an 11-year battle for justice by her mother.
The teenager’s death caused public outrage among the millions of tourists who visit Goa’s beaches.
A trial court acquitted both of the accused in 2016, saying there was not enough evidence to convict them.
The High Court took up the case in 2017 after an appeal by the Central Bureau of Investigation, India’s FBI.
Under Indian laws, both the accused and prosecutors can appeal against a trial court’s verdict to the High Court and the Supreme Court.
D’Souza and prosecutors can still appeal against his sentence to the Supreme Court, and prosecutors can still appeal against the acquittal of the second suspect as well.