The United Nations earlier said the water blockage was a possible war crime as the internationally-recognised government blamed the closure on forces loyal to Khalifa Haftar, who has been trying to capture Tripoli.
Authorities say the Libyan capital, which has been under attack by commander Khalifa Haftar's forces since last month, has seen its water supplies resume two days after gunmen shut the pipes down, depriving over 2 million residents of water.
The interior ministry says the men on Sunday stormed the offices of a water distribution agency that runs a network of underground pipelines providing the capital and the region with water and shut it at gunpoint.
The ministry says the armed men demanded the internationally-recognised government release their leader Khalifa Ahnish's brother, jailed for belonging to an outlawed group.
The UN humanitarian coordinator, Maria Ribeiro, condemned the attack and said that such actions "may be considered war crimes."
The water distribution agency says water supplies to Tripoli resumed on Tuesday, without elaborating.
Meanwhile, Libya's state oil firm NOC is concerned about the country's lawlessness affecting the oil industry, it said on Tuesday.
Equipment and machinery had been stolen at the headquarters at the NAGECO exploration firm, an NOC unit, it said in a statement.
In another security incident a fuel truck was seized in southern Libya, it added.
Fighting in the battle for Tripoli has killed at least 510 people, forced 75,000 out of their homes, trapped thousands of migrants in detention centres, and flattened some southern suburbs.
It has also forced the closure of schools, split families on different sides of the front line, and brought power-cuts.
The conflict is one of the most serious flare-ups in years of chaos since the 2011 toppling of Muammar Gaddafi, and has sharpened Gulf divisions over Libya.