The move by the Basque group mirrors the decommissioning process in Northern Ireland, where nationalist and loyalist paramilitaries disposed of their weapons in front of independent witnesses as part of a peace deal ending decades of violence.
The head of the international commission, Ram Manikkalingam, said: "We have verified that ETA has put out of operative use a certain amount of arms and explosives. We believe this is a credible and significant step."
The commission showed journalists a three-page inventory of the weapons handed over, which included two rifles, two revolvers, 300 rounds of ammunition, two hand-grenades, 16kg of chemicals for making explosives, a detonator cable and timers.
ETA announced a unilateral ceasefire two years ago, suspending four decades of attacks in which more than 800 people died.
The group has been severely weakened by hundreds of arrests and flagging support in the north-western Basque region.
In 2012, ETA said it wanted talks with the Spanish and French governments, to negotiate a definitive end to military operations and a handover of its arms.
But Spain's then newly elected conservative government, led by the People's Party's Mariano Rajoy, rejected the offer and demanded that ETA dissolve itself without any conditions.
Interior Minister Jorge Fernandez Diaz welcomed the handing in of arms as positive, but said that if the group really wanted to disarm they could just give police the location of their arsenal, without needing international monitors.
He said: "The only thing Spanish society wants is the definitive dissolution of ETA, without conditions and concessions and that's what the state is working towards."