Garcia scored on a power play with 46 seconds to go, rising out of the water to flip a pass from captain Jennifer Pareja past U.S. goalkeeper Betsey Armstrong and inside the near post. The goal capped a furious rearguard action for Spain after trailing by three goals with less than three minutes to play.
"I didn't look at the time, I didn't know how much time was left, I just thought if I get the ball and I see a gap, I'll shoot. I said 'no pressure, I'll just go for it,'" Garcia said.
The Spanish, who are making their Olympic debut in women's water polo, controlled the first half and led 5-3 at the break, with captain Jennifer Pareja scoring three goals.
But a veteran U.S. team improved in second half, outscoring the Spanish 6-1 over to lead 9-6 with three minutes to play.
Undaunted, Spain roared back with Pareja's fourth goal and then two from Garcia to earn the draw.
"It's hard to hold Spain down for so long," U.S. coach Adam Krikorian said. "We played great defensively in the second half, besides the last two minutes."
The draw left both teams level atop Group A with three points from two games.
The U.S. has been on the podium at every Olympics since the women's game debuted in 2000, finishing third in 2004 and second in 2000 and 2008. With seven players back from the Beijing squad, the Americans are the gold medal favourite in London along with rival Australia.
Spain, meanwhile, is a newcomer to the sport's marquee competition, and can draw confidence from Wednesday's draw.
"This means that we're there with the best, our hard work is paying off. We're playing really good water polo," Garcia said. "We want the respect of other teams, we're a young team but I do think now that the other teams respect us because we showed them what we're capable of."
In the group's other game, Hungary bounced back from its opening loss to the U.S. to earn a hard-fought 11-10 win against China and pick up two points in Group A.
With the game evenly poised at 10-10, Barbara Bujka outmuscled China's Ma Huanhuan to fire a shot past goalkeeper Yang Jun with under two minutes to play and give Hungary its first — and only — lead of the game.
"We work a lot to get to this level, and this was the right time to show how we can play and how strong we are together," said Gabriella Szucs, who led Hungary with four goals.
The loss was the second straight for China, the 2011 world championship runner-up, and left them at the bottom of the group.
In Group B, Ekaterina Lisunova scored three goals as Russia beat 2012 European champion Italy 7-4, while Webster scored five as Australia crushed Britain 16-3.
"I guess you have to take your opportunities and there's a hole there and you just have to pop them in," Webster said. "We kind of take it in turns who scores goals on what days, and I was just lucky enough to bag five."
Australia and Russia lead the group with four points apiece, while Britain and Italy are both at the bottom with zero.
The eight-team field in women's water polo is broken down into two groups of four. All eight teams advance to the quarterfinals, but the results from the preliminary stage determine the team's seeding for the knockout stage.
Ryan Lucas can be reached on twitter at www.twitter.com/relucasz