The oil giant will take a stake in a new engine company set up by the two automakers.
That follows Reuters reports in January that Aramco would have 20% of the new firm.
On Thursday (March 2) it wasn't disclosed how much Aramco would invest, but Renault and Geely are expected to have equal stakes.
The new joint venture is set to develop more efficient internal combustion engines and hybrid technologies.
It will have the capacity to make more than five million engines and transmissions per year.
By carving out the gasoline engines business, Renault hopes it will be left free to focus on electric cars.
The move is part of the French firm's broader restructuring, which also involves a reset of its long alliance with Nissan.
Chief executive Luca De Meo said Aramco's involvement would help drive breakthroughs in synthetic and hydrogen fuels.
It makes the Saudi firm the first big oil producer to invest in a car business, at a time when the rise of EVs threatens to cut demand for fuels.
For Geely, the deal extends its pattern of building partnerships to expand overseas.
It has previously announced a tie-up with Mercedes Benz on hybrid engines.
