Spanish Defence Minister Jose Bono is to travel to Venezuela next week to oversee the signing of an arms contract that is opposed by the US.
The deal was reached during Mr Zapatero's visit to Venezuela
Madrid has agreed to sell military patrol boats and transport planes to Caracas in deal worth more than $1.5bn.
The two countries insist the equipment is for peaceful purposes, such as to help in the fight against drug gangs.
But the US regards Venezuelan President Hugo Chavez as a destabilising influence in the region.
US ambassador Eduardo Aguirre had asked the Spanish authorities not to go ahead with the sale.
He said Washington was considering whether to allow Spain to sell aircraft made with US technology, which would require a US export licence.
Spain's Deputy Prime Minister, Teresa Fernandez de La Vega, confirmed Mr Bono's visit to Caracas.
But she insisted the deal had been negotiated with "scrupulous respect for international law".
Mr Chavez said the contracts would be signed on Monday, according to the Spanish newspaper ABC's website.
The arms deal was agreed during Spanish Prime Minister Jose Luis Rodriguez Zapatero's a two-day visit to Venezuela last March.
President Chavez said at the time that the boats would be used to step up Venezuela's coastal patrols against the drugs trade, while the transport planes would be used mainly for humanitarian missions inside and outside the country.
Tensions between Washington and Caracas have grown in recent months, in part because of US criticism over Venezuela's purchases of military equipment.