Kurdish elements of the Syrian Democratic Forces (SDF) would be equipped to help drive IS from its stronghold, Raqqa, a spokeswoman said.
The US was "keenly aware" of Turkey's concerns about such a move, she added.
Turkey views the Kurdish rebels as terrorists and wants to stop them taking more territory in Syria.
The Pentagon later said US Defence Secretary Jim Mattis had spoken by phone to his Turkish counterpart, Fikri Isik, but gave no details of the conversation. Turkish officials have not yet responded publicly.
SDF forces, which comprise Kurdish and Arab militias, are already being supported by elite US forces and air strikes from a US-led coalition.
The US has previously supplied light weapons and armoured vehicles to the Arab element of the SDF, known as the Syrian Arab Coalition.
The SDF is currently battling IS for control of the city of Tabqa, an IS command centre just 50km (30 miles) from Raqqa.
The Kurdish fighters are from the Kurdish Peoples' Protection Units (YPG), which Turkey sees as an extension of the outlawed Kurdistan Workers Party (PKK), a group it has been fighting for decades.
Last month, Turkey carried out air strikes on YPG positions in Syria which it described as "terrorist havens".
The US believes the Kurdish fighters will be essential to Raqqa's downfall.
The Pentagon sees them as the most disciplined and organised of the anti-IS groups but Turkish opposition has meant Washington has had to tread a fine line.
The imminence of the fight for the city means delay is no longer an option and the Kurds will be getting a range of equipment.
US sources say they have received assurances from the Kurds that they will leave Raqqa to be governed by Syrian Arabs after the battle.
Turkey's President Recep Tayyip Erdogan is due in Washington next week - he will not be a happy visitor.
A Pentagon source told the BBC the equipment would include ammunition, small arms, machine guns, heavy machine guns, construction equipment such as bulldozers and armoured vehicles.
The source added that the US would "seek to recover" the equipment afterwards.
No timeline has been given for when the weapons would start to be supplied.
"We are keenly aware of the security concerns of our coalition partner Turkey," said Pentagon spokeswoman Dana White, who is travelling with Mr Mattis in Lithuania.
"We want to reassure the people and government of Turkey that the US is committed to preventing additional security risks and protecting our Nato ally."
US President Donald Trump has approved supplying weapons to Kurdish forces fighting so-called Islamic State (IS) in Syria, the Pentagon says.