The convicted IS members received the death penalty for murdering 53 people and destroying public property in the western city, according to a statement. Two of the 16 other militants were given life terms in prison.
The court did not specify when the sentences would be carried out.
Libya remains split between two rival administrations after years of civil war with the divide between authorities in the capital of Tripoli and eastern Libya leading to widespread lawlessness.
The IS extremist group expanded their reach in Libya after the 2011 uprising that toppled and later killed longtime ruler Moammar Gadhafi. Militants first seized the city of Darna in 2014 and then later Sirte and areas surrounding the city of Sabratha.
Unlike in Syria and Iraq, IS was unable to profit from chaos and take large swathes of Libya.
Instead, the group was limited to only administrative pockets dotted across the oil-rich North African country, unable to gain supremacy over Libya's numerous well-armed militia forces tightly bound by tribal loyalties.
Several IS training camps were located outside Sabratha. In early 2016, some 700 of its fighters, most of who were Tunisian, were based in the area.
In March 2016, affiliates of the group briefly took over the city's security headquarters and beheaded 12 Libyan security officials before using the headless corpses to block nearby roads.
Sirte’s central Martyrs’ Square was transformed by IS into a stage for public extrajudicial killings — including beheadings by a sword — for a wide variety of offenses.
In April 2016, near the height of its power, the Libyan branch of the militant group had recruited around 6,000 fighters, U.S. military experts estimated.
IS was driven from its main stronghold, the coastal city of Sirte, in late 2016 and fled inland. However, the militants maintain a limited presence in small pockets of the country including the areas surrounding Sabratha.
In February 2016, the United States carried out an airstrike on an IS training camp near Sabratha, killing at least 40 people, as part of its effort to eradicate the group.
