General Joseph Votel, head of US Central Command, spent about 11 hours in northern Syria.
He met US military advisers and the leaders of the Syrian Democratic Forces (SDF), made up of Kurdish and Arab rebel forces.
The US wants local forces to defeat the so-called Islamic State (IS) group, which holds territory in the country.
Speaking after the visit, Gen Votel said training local forces to fight IS was the right approach.
"I left with increased confidence in their capabilities and our ability to support them. I think that model is working and working well," he said.
The SDF comprises about 25,000 Kurdish fighters and about 5,000 Arab fighters. The US is hoping to increase the number of Arabs in the force.
Arab commanders who spoke to journalists during the visit said their forces needed more help.
SDF Deputy Commander Qarhaman Hasan said he wanted armoured vehicles, machine guns, rocket launchers and mortars.
The SDF currently had to rely on smuggling to get weapons, he said.
"You can't run an army on smuggling," he added.
Tribal leaders also called on the US to do more, both militarily and with humanitarian aid.
The US has about 200 military advisers in Syria, where 270,000 people have died in five years of civil war.
The top US commander for the Middle East secretly visited Syria on Saturday, officials said.