Zia Vessey, 22, has a previous conviction for the same charge and was on probation for that when he committed one of the offences.
He was convicted of having unlawful sex with a 14-year-old girl at a house in Lerwick in 2010 and with a 15-year-old girl in 2012.
Vessey was jailed at the High Court in Glasgow.
The trial had been held at the High Court in Aberdeen, where Vessey denied the charges.
A man convicted of having underage sex with two teenage girls in Shetland has been jailed for three years.