The Al-Omar oilfield in the province of Deir Ezzor produced 30,000 barrels per day before the start of Syria's conflict in 2011 and became a key source of income for the jihadists after they seized it in 2014.
US-led coalition air strikes destroyed the field in 2015, after the jihadists had reaped estimated oil sale revenues from it of between $1.7 million and $5.1 million a month, according to the coalition.
"The Syrian Democratic Forces seize the whole of the Al-Omar oilfield, the biggest field in Syria," the alliance said in a short statement.
It said regime forces stood three kilometres (less than two miles) away from the field.
The SDF and Russia-backed government forces are waging separate offensives against IS in the oil-rich province of Deir Ezzor on Syria's eastern border with Iraq.
The Syrian Observatory for Human Rights, a Britain-based monitor relying of a network of sources inside Syria, said SDF fighters took control of Al-Omar three days after IS members retreated.
Its capture came after the jihadists led "a counterattack on regime positions near the field late Saturday, pushing them away from it," Observatory head Rami Abdel Rahman said.
Al-Omar lies on the eastern bank of the Euphrates River, around 10 kilometres (six miles) east of the town of Mayadeen.
Government forces and their allies seized Mayadeen from IS last week in an advance whose target the Observatory said was to recapture Al-Omar.
On Sunday, the monitor said the SDF had also seized the Sayjan oilfield to the north of Al-Omar overnight.
Deir Ezzor province is rich with oil and gas fields that served as a key revenue stream for IS at the height of its power.
The SDF, which earlier this week forced IS from its former stronghold Raqa, has been fighting the jihadists on the eastern bank of the Euphrates.
Syria's army is carrying out a separate operation mostly on the western bank of the river, including in the provincial capital Deir Ezzor city.
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