Reese, gold medallist in 2009 and 2011, had scraped into the final on countback after a below-par showing in Saturday's qualifying but the American had no intention of losing her crown and, after opening with a foul, leapt 7.01 metres.
It was the only seven metre jump of the competition and left Nigeria's Blessing Okagbare, who briefly led in the first round with 6.89 and later improved to 6.99 on her penultimate jump, with silver.
Serbia's Ivana Spanovic was third with a national record 6.82 metres on countback from Belarussian Volha Sudarava.
Before celebrating, Olympic champion Reese put on a t-shirt bearing the words 'unleash the beast', hugged Okagbare and then holding the Stars and Stripes flag, posed for photographers.
Olga Kucherenko in fifth was the best of the three Russians in the final.
(Writing by Alison Wildey; Editing by Justin Palmer)
