In a since-deleted post, an anonymous social media user said a girl who is seen with Giddey in videos and photographs was a high school junior at the time. The social media account has since been deactivated.
The NBA confirmed on Friday they were investigating the claim, with spokesman Michael Bass saying: "We're looking into it."
As a result, the 21-year-old Oklahoma City Thunder guard's career now hangs in the balance.
Giddey had no comment when questioned at practice on Friday.
"Yeah, I mean, I understand the question, obviously," he said. "But there's no further comment right now."
When asked again, he said: "I get the question guys. I completely understand you guys want to know about it. But just for right now, I don't have anything to say."
One of Australia's most high-profile and best paid athletes, Giddey is viewed as the shooting star key to the Boomers' Paris Olympics medal chances.
Giddey, the son of Melbourne Tigers great Warwick, was named NBL rookie of the year with the Adelaide 36ers in 2021 and made a phenomenal start to his NBA career with the Thunder.
He was the youngest NBA player to record a triple-double - at 19 years and 84 days - and the first player since Hall of Fame member Oscar Robertson in 1961 to record three consecutive triple-doubles as a rookie.
Over his first two seasons, Giddey averaged 14.9 points, 7.8 rebounds and 6.3 assists per game.
He improved his scoring output from 12.5 points per game in his rookie season to 16.6 points per game last season, and top-scored for Australia while running the point at this year's FIBA World Cup.
Giddey's stunning early-career success prompted Boomers coach Brian Goorjian's prediction on the eve of the young star's third NBA campaign that his stock would continue to rise.
"He's an amazing talent," Goorjian told AAP in October.
"He can play at his pace. You can't speed him up.
"He's absolutely gifted at passing the ball. This (Thunder) is a really good team for Giddey.
"He's a player the other players are excited to play with, because he makes them all better, and that's a sign of greatness."
Giddey had made a modest start to the current NBA season as a support act to back-court partner and Oklahoma City superstar Shai Gilgeous-Alexander.
The Australian guard has averaged 12.3 points, 5.7 rebounds and 4.5 assists in 15 games for the 11-4 Thunder, who sit second in the Western Conference standings with the league's third-best record.
