The 25-year-old finished the 200-meter breaststroke in 2 minutes 19.59 seconds.
"I'm so happy," Soni said. "I didn't try to focus on medal or records, I just wanted to swim one more race the way I knew I could...I just wanted my hand on the wall. I was scared to look. I can't believe it yet."
It has been a personal goal of hers to swim under 2:20 dating back to her days at West Windsor-Plainsboro High School North in Plainsboro Township, N.J. Soni's coach then predicted she would be the first under that barrier.
Soni has another chance for gold at the London 2012 games in the 4x100 medley relay.