The Americans have won every game on their way to the gold medal match at Wembley, but needed a 122nd-minute header from Alex Morgan to dramatically beat fierce rivals Canada in a thrilling semi-final.
Japan were less impressive during their group campaign, being held to two draws, but have since shown their credentials with victories over Brazil and France in the knockout stages.
They possess the tightest defence in the competition, while the US can boast of the most prolific attack, which is led by Morgan and Abby Wambach.
Both of those are expected to start tomorrow in what is a rematch from the World Cup final of 2011, with the US looking to gain some revenge and defend their title from Beijing 2008 in the process.
Japan: Fukumoto; Kinga, Iwashimizu, Kumagai, Sameshima; Sakaguchi, Miyama, Kawasumi, Sawa; Ohno, Ogimi
United States: Solo; Rampone, O'Hara, Le Peilbet, Buehler; Lloyd, Rapinoe, Heath; Cheney, Morgan, Wambach
Sports Mole says: 2-1 (AET)