Ahn won three golds and a bronze at the Turin Games of 2006 while representing his native South Korea. He now has two more medals at the Sochi Games as a citizen of Russia. Ahn won bronze earlier in the Olympics in the 500-meter race.
In the other short-track final held Saturday, Zhou Yang of China successfully defended her Olympic title over 1,500 meters.
Ahn edged Russian teammate Vladimir Grigorev by .074 of a second to earn the title. It was the third gold of the Olympics for the host country.
Sjinkie Knegt of the Netherlands won the bronze. Until Saturday, all of the 12 medals won by the Netherlands at these Games had come in oval speed skating.
American J.R. Celski fell in his quarterfinal heat and again was shut out of medal contention.
In the women's race, seven competitors were placed in the finals after various penalties had cost skaters a fair chance to compete. That made for a crowded track and some sort of incident was almost inevitable.
Sure enough, three skaters hit the ice early in the race in an incident the judges decided had been caused by South Korea's Alang Kim. One of those caught up in the crash was American Emily Scott.
That left four skaters to compete for the medals and Zhou won by a tenth of a second over Suk-Hee Shim of South Korea.
Arianna Fontana of Italy took the bronze for her second medal of the Olympics. She finished second in the 500-meter race.