Kurds raised their banners in place of black IS flags, celebrating their victory over the jihadist group.
The capture of Tal Abyad gives them control of hundreds of kilometres of territory along the Turkish border.
Activists the Syrian Observatory for Human Rights said it was IS' "biggest setback" since announcing a caliphate.
Tal Abyad was a major supply route for the group, acting as a gateway to its headquarters at Raqqa.
"The whole city is under our control and there is no more fighting," Huseyin Kocher, a Kurdish YPG commander in Tal Abyad told the BBC.
"Our people should know that we are going to clean all the remnants of IS in northern Syria."
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The Kurdish Popular Protection Units (YPG) have been supported by Syrian rebel groups and air strikes by the US-led coalition against IS.
Days of fierce clashes around Tal Abyad prompted more than 16,000 civilians to flee their homes and cross the border with Turkey.
Sherfan Darwish, a spokesman for the Burkan al-Furat rebel group fighting alongside the Kurds told AFP there were "mines and car bombs everywhere".
"The bodies of IS fighters are lying in the streets," he added.
YPG fighters have been moving into IS territory on either side of Raqqa province since forcing the jihadists to withdraw from Kobane in January.
Their advance has prompted criticism from Turkish officials, who fear the  growing power of Kurdish forces in Syria will embolden Turkey's Kurdish minority.
On Monday, a group of 15 Syrian rebel factions accused the YPG of carrying out a "sectarian and ethnic cleansing campaign" against Arab and Turkmen civilians from Tal Abyad and the western countryside of neighbouring Hassakeh province as it advanced into IS territory.
YPG spokesman Redur Khalil strongly denied the allegation, calling the rebel factions "bankrupt".
Syrian Kurdish fighters say they are clearing mines and booby traps from the border city of Tal Abyad after seizing it from Islamic State militants.