It was the third straight title for the United States and the fourth in the five Olympics in which soccer has been played by women.
Lloyd scored the goal in extra time that gave Team USA a 1-0 win over Brazil in the title game in Beijing four years ago and she was the star again in 2012. She gave the Americans the lead 8 minutes into the game when she sidestepped a defender and scored from point-blank range.
Lloyd sent a header into the left corner of the net in the 54th minute to give the Americans a 2-0 lead.
Japan, which won last year's World Cup in a penalty shootout, clawed back in the game on a goal by Yuki Ugimi in the 63rd minute due to a defensive lapse and Solo then came through with the game-saver.
Mana Iwabuchi broke in all alone in the 83rd minute and her shot headed for the right side of the net. Solo lunged in that direction and got just enough of the ball to deflect it wide of the goal.
Solo also made a big save in the first half with a deflection that bounced off the crossbar.
The game attracted 80,203 to Wembley Stadium, a record for a women's Olympic soccer game.