The 36-year-old Solo, her country's standout goalkeeper, claimed the 81-year-old Blatter pounced as she was about to present an award to her teammate Abby Wambach.
"Sepp Blatter grabbed my ass, it was a few years ago at the Ballon d'Or ceremony just before I got on stage," Solo told Portuguese newspaper Expresso.
A spokesman for Blatter, however, said the accusation was "ridiculous".
Solo, a World Cup winner and two-time Olympic champion, made the allegations on the sidelines of the Web Summit being held in Lisbon.
She said that sexual harassment at the hands of male officials was a common problem in women's football.
"I have seen this all of my career and I would like to see more athletes speak about their experiences," said Solo.
"It's out of control, not just in Hollywood but everywhere," she added in reference to the firestorm of sexual harassment allegations in entertainment, politics and sport ever since the scandal involving Hollywood mogul Harvey Weinstein unfolded.
Blatter was president of FIFA from 1998 until 2015 until he was banned for corruption.
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