Robbie Williams (Getty Images)
The former frontman of English boy band Take That raised his finger after singing his track "Rock DJ" before the global audience in Moscow on Thursday.
The 44-year-old did not explain his gesture but he appeared to mouth the phrase "I did this for free" - a response to criticism for gracing an event dear to Russian president Vladimir Putin.
"The 2018 FIFA World Cup opening ceremony was a newsworthy event produced by a third party and carried live on Fox," Fox Sports said in a statement.
"As it was broadcast live, we did not know what would happen during Robbie Williams' performance and we apologise," it said.
The middle finger, while innocent by the standards of pop star antics on stage, prompted a deluge of upset comments on social media, many from parents saying the gesture was inappropriate for children.
After repeated incidents, US networks routinely broadcast domestic events with a short delay to allow time to cut out incidents that may offend viewers.
Networks have become even more fastidious since the 2004 Super Bowl, the most watched event on US television, when Justin Timberlake accidentally bared Janet Jackson's nipple while dancing, scandalising audiences.
Hip-hop star M.I.A. nonetheless gave a middle finger during the 2012 Super Bowl, leading the National Football League to take legal action against her for violation of its contract.
Williams sang in Moscow alongside Russian soprano Aida Garifullina, with the two singing a duet of his hit solo song "Angels."