Maggie Steffens scored five goals for the USA as they cruised past the Europeans in the tournament final.
The win gives the USA their first Olympic Water Polo gold, after silver at Beijing 2008, bronze at Athens 2004 and silver in the first women's Water Polo competition at Sydney 2000.
A flurry of seven unanswered goals across the second, third and fourth periods propelled the USA from 2-1 down into a commanding position, 8-2 up, in the final quarter.
Superb shooting and resilient defensive work by the USA kept their scoreboard ticking over, while denying their opponents vital chances.
A stunning performance from the tournament's 'All-Star' goalkeeper, Betsey Armstrong, kept the Spaniards from scoring the goals that, until tonight, had given them an unbeaten record.
Spain did rally to outscore their opponents in the final quarter, but they left it too late to seriously bother the eventual winners.
After the game, Steffens deflected the praise for her performance and insisted the credit should be shared among the USA team.
'Definitely it was from my team-mates. When you're getting up there it's about the pass, it's about the steal and our defensive blocks were really creating the offensive end for us,' said Steffens.
'And you could tell towards the second half the attacking side slowed down, but the defence really started to pick up, so I think that was really important.'
Asked when the gold medallists felt they had won the game, she added: 'We didn't feel like we had it in the bag until the final buzzer rings. We've learned that.'
Earlier, Australia took bronze with a dramatic extra-time win over Hungary.
In a repeat of the Beijing 2008 third-place play-off, the Australians looked to have won their second successive bronze in normal time.
A last-second Hungarian leveller forced the tight game into added time, but Australia scored two unanswered goals to clinch the match 13-11.