The SDF forces were replaced by Turkish-backed rebels who had previously besieged and shelled the town, the watchdog told dpa.
SDF spokesman Kino Gabriel confirmed the withdrawal from the town.
"As part of the agreement to pause military operations with Turkey with American mediation today, we have evacuated the city of Ras Al-Ain from all SDF fighters. We don't have any more fighters in the city," Gabriel tweeted.
On Thursday, US Vice President Mike Pence said Turkey would pause for 120 hours an offensive it launched in north-eastern Syria on October 9, to allow Syrian Kurdish fighters to withdraw from a "safe zone" sought by Ankara.
Turkish and Kurdish forces have since accused each other of breaking the arrangement.
Turkey insists SDF leave a 444-kilometre-long, 32-kilometre-deep safe zone along its border with Syria. Ankara considers the SDF to be terrorists linked to insurgents at home.
On Sunday, US Secretary of State Mike Pompeo said there has been "relatively little fighting" in violation of the truce and that he's optimistic about the situation.
He described the violence to US broadcaster ABC as "a little sporadic small arms fire, a mortar or two."
The five-day ceasefire is to continue through Tuesday under the agreement. If the withdrawal is completed by then, Turkey is to halt is operations and the US will lift recent sanctions imposed on Turkey.
Asked about worries about the ethnic cleansing of the Kurds by Turkey, Pompeo said the ceasefire agreement says that in Turkish-controlled space there "wouldn't be attacks on minorities."
Pompeo, who was went to Turkey with Pence to negotiate the ceasefire, also pushed back against Turkey's claim that it got everything they wanted in the agreement.
"I was there. It sure didn't feel that way when we were negotiating," he said, adding that he and Pence achieved the outcome that President Donald Trump sent them to achieve.
On Sunday, Turkey sharply rejected accusations from Amnesty International of committing war crimes in northern Syria during the incursion, dismissing them as being part of defamation campaign against Turkey's fight against terrorism.
In a report published on Friday, Amnesty accused the Turkish army and its allies of killing and injuring civilians in "ruthless attacks on residential areas" in the Syrian region.
The Foreign Ministry in Ankara said its operation in Syria was aimed only at positions and hiding places of the Kurdish YPG militia and that it was taking all necessary precautions to ensure that civilians were not harmed.
According to research from the human rights watchdog, pro-Turkish rebels had also shot a Syrian-Kurdish politician.
Previously responding to accusations of war crimes in Syria, President Recep Tayyip Erdogan said on Friday that some rebels had made mistakes and promised to "take care of the matter."
The Defence Ministry in Ankara said Sunday that a Turkish soldier had been killed and another injured by YPG fighters in the border area of Tal Abyad in a breach of the US-brokered ceasefire.
The soldiers were shot with anti-tank weapons during a "reconnaissance and surveillance mission."
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