Stuart Richard Pike, 45, was found guilty of molesting the 17-year-old in April last year.
The Briton had maintained his innocence, saying he did not know the victim was under the legal age of consent -- 18 -- and he thought the encounter was consensual.
Presiding judge Didiek Jatmiko told a court on the island that Pike was guilty of "violence against children and molestation", as he handed down the sentence.
As well as the jail term, Pike -- who has lived on Lombok for 11 years -- was ordered to pay 60 million rupiah ($4,500), or to spend another three months in jail if he was unable to pay the fine.
Pike's lawyer said he was considering whether to file an appeal.
There are regular cases of foreigners sexually abusing minors in Indonesia, particularly on Lombok and the neighbouring resort island of Bali.
In October last year, a 70-year-old Australian man was jailed for 15 years for sexually abusing 11 Indonesian girls on Bali after luring them to his house with promises of gifts.