Nadal and Alcaraz win Olympics men's tennis doubles opener
In front of a packed, raucous crowd under the roof on Court Philippe Chatrier the pair came through 7-6 (7/4), 6-4.
Hours earlier when Alcaraz had opened his singles campaign with a straights sets victory over Hady Habib, the 21-year-old said it was a "dream" to play alongside his compatriot.
The duo appeared to a standing ovation on the same packed Court Philippe Chatrier where Nadal won 14 French Opens and Alcaraz collected his first just last month.
Nadal, the winner of Olympic singles gold in 2008 and doubles eight years later in Rio, had his right thigh bandaged, the legacy of an injury which could yet torpedo his singles hopes where old rival Novak Djokovic looms as a second round opponent.
Gonzalez and Molteni, the sixth seeds, entered the arena to a chorus of boos in the aftermath of a recent racism row between Argentina and France.
The Spanish pair endured a nightmare start when Alcaraz was broken in the first game. Having not played doubles on tour since 2022, the rustiness was not surprising.
The greater experience of Nadal was key as the break was quickly retrieved with the 38-year-old's reflexes as razor sharp as ever at the net.
Alcaraz and Nadal went to three set points in the tiebreaker and a pinpoint down the line backhand by Nadal secured the opener.
Gonzalez and Molteni raced to a 3-0 lead in the second set but back came the Spanish pair to level.
Nadal secured another key break with a rasping backhand return to allow him and his partner to lead 5-4 and the opportunity to serve for the match.
That honour fell to the 22-time Grand Slam title champion and victory was secured when Alcaraz slapped the winning forehand crouching at the net.