The agreement, signed by Russian Deputy Prime Minister Boris Alyoshin and French Prime Minister Jean-Pierre Raffarin, gives the legal green light to using the French overseas territory for ESA-Russian cooperation.
Raffarin hailed the accord as a "great step in economic and scientific cooperation," while Alyoshin called it a "step forward in relations between Europe and Russia."
ESA ministers agreed to the scheme in May. It provides for the building of a new pad at Kourou, at a cost of 314 million euros (361 million dollars), from which to launch the veteran Soyuz rocket.
France has agreed to contribute half of the costs, with other ESA member states to pick up the rest of the tab.
Arianespace, which operates ESA's launchers, will tie up with Russia's Starsem company to use the Soviet-era Soyuz for launching medium-sized payloads to help meet a gap in its own marketing range.
The Soyuz -- the workhouse of space, having been used on nearly 1,700 satellite launches or manned space missions -- is expected to substitute for the Ariane-4 rocket, which was phased out earlier this year.