The former UFC champion denies the allegations, which are being investigated by police in Miami.
Image source, ReutersUFC fighter Conor McGregor has been accused of sexually assaulting a woman after an NBA Finals basketball game in Miami earlier this month.
A legal letter outlining allegations, sent to Mr McGregor and seen by BBC News, states the alleged assault took place in a bathroom at Kaseya Center.
A lawyer representing Mr McGregor said: “The allegations are false. Mr McGregor will not be intimidated.”
The City of Miami Police said an investigation had been opened.
In a statement, UFC said it was aware of the allegations and was gathering further details. The Miami Heat, which hosted the NBA Finals game, said it was conducting a full investigation.
The woman’s lawyer, Ariel Mitchell, said her client had been watching game four of the NBA Finals on Friday 9 June before the alleged incident happened.
In the legal letter to Mr McGregor, Ms Mitchell alleged the woman was forced into a men’s bathroom by security guards from the NBA and Miami Heat before being violently sexually assaulted by Mr McGregor.
The letter claimed the woman was able to free herself from the bathroom, but left behind her purse, which she is said to have retrieved after pleading with security guards.
It alleges that security for the league, team and arena “aided and abetted” Mr McGregor by trapping and isolating her in the bathroom.
In an interview, Ms Mitchell said she had obtained video footage showing part of the alleged incident.
A police report relating to the incident was made on Sunday 11 June, the City of Miami Police told BBC News.
“This is an open investigation so no additional information can be released at this time,” a police department spokeswoman said.
Mr McGregor’s appearance at the NBA Finals made headlines after the Irish former champion hit the Miami Heat’s mascot during half-time, in an apparent stunt for a pain relief spray he was promoting.
The person acting as the mascot was taken to a local hospital for treatment after the incident, the Athletic reported.