Poll went one better than her sister Silvia, who took the silver medal in the event at the 1988 Seoul Olympics.
She fully deserved her victory over the German favourite. Van Almsick, silver medallist behind American Nicole Haislett at the 1992 Barcelona Games aged just 14, seized the lead on the first length but Poll had caught her by the 100-metre mark when she had the edge by 0.02 of a second.
The 23-year-old Costa Rican, bronze medallist behind the victorious van Almsick at the 1994 world championships in Rome, pulled further clear on the third length of the Georgia Tech pool and held her advantage to win in 1min 58.16sec.
Van Almsick had to settle for her second Olympic silver in 1:58.57. Her fellow former East German Dagmar Hase, the Olympic 400 freestyle champion, took the bronze in 1:59.56, just 0.01 of a second ahead of the American Trina Jackson.
Penny Heyns, the South African who lowered 100m breaststroke world record to 1min 07.02sec in qualifying for the final, duly won the gold medal but she could not improve on her time from the heats. She held off the 14-year-old American Amanda Beard to win in 1:07.73.
The host nation finally collected its first gold medal when the world champion Tom Dolan won the men's 400m individual medley title after a duel with his compatriot Eric Namesnik. It was not until the final length that Dolan eased ahead.
Canada's Curtis Myden, who led on the opening butterfly leg, took the bronze.
Britain's men's 4 x 200m team swam superbly to finish fifth a British record time of 7:18.74 in an exciting final won by the United States, in 7:14.84, ahead of Sweden.