The bomb factory is the largest to be captured by fighters allied with the Tripoli-based Government of National Accord (GNA) since they launched an operation to retake Sirte in May, the forces said on Facebook.
They said the building was in the southeast of Sirte, the hometown of slain dictator Moamer Kadhafi which IS has controlled since June 2015.
The pro-GNA forces shared images of the building including several rooms containing explosive devices, cables and mobile phones.
The fall of Sirte, 450 kilometres (280 miles) east of Tripoli, would be a major blow to IS, which has faced a series of setbacks in Syria and Iraq.
The two-month battle for Sirte has killed around 280 pro-government fighters and wounded more than 1,500, according to medical sources at the unity forces' command centre.
The pro-GNA forces are mostly made up of militias from western Libya established during the 2011 revolt that overthrew Kadhafi.
A militia set up to guard the country's main oil facilities has also been advancing on IS.
The GNA was the result of a UN-brokered power-sharing agreement struck in December, but it has yet to be endorsed by Libya's elected parliament based in the country's far east.
A fighter of Libyan forces allied with the UN-backed government fires a weapon during a battle with IS fighters in Sirte, Libya, on July 21, 2016.
Forces loyal to Libya's unity government said Sunday they had seized a building used by the Islamic State group to manufacture explosives in its coastal stronghold.
