Genevieve Pascolla, 26, who was travelling from London’s Heathrow to Chicago, took to Instagram to share a video of the incident and the response she got from United Airlines after sending them a formal complaint.
She wrote: "On this flight the man next to me started masturbating. I woke up and saw him touching himself under a blanket. I woke the woman next to me and got out of there to tell a flight attendant, the attendants checked and confirmed that he was masturbating in public.
"They then started making jokes about the situation asking 'what perfume are you wearing' and excusing him saying 'he’s had a bit of wine' no one stopped him."
She continued: "He was allowed to finish, with a child sitting closely by. After exiting the plane first to talk to security the security guard asked me if I’d like an apology from the offender I immediately rejected the request."
Genevieve, who works as a photographer, said the incident happened when she was travelling home to Chicago on 4 June.
In an email to Genevieve, United Airlines’ Customer Care Officer Lauren Michaels said: "Your observation are important as they help us improve areas needing our attention.
"Unfortunately, we are unable to honour your request for a refund of your travel from London to Chicago or offer goodwill compensation.
"Again, I apologize for the uncomfortable situation you experienced. I appreciate that you chose United for your travel. We hope your next trip with us will be a more positive experience."
In reply to the mail from United Airlines, Genevieve said: "It took United Airlines a month an a half to respond to my complaint. And I received this email today. I am appalled at the lack of action taken in this situation in which as a woman, I was terrified.
"This man is clearly capable of much more. So women and men out there pick an airline that cares about your safety."
She concluded: "United Airlines referring to being assaulted on one of your flight as an 'uncomfortable experience' is a bit of an understatement. (See email) your lack of action is disgusting."
In a statement to BuzzFeed News, United Airlines spokeswoman Maddie King called the incident "inappropriate and offensive" but said that incidents like these are "extremely rare".
She said: "That’s why, in this case, our customers were promptly moved to different seats in a different section of the plane and law enforcement officials were summoned in advance to meet the perpetrator when the plane pulled into the gate."
Watch the offensive video below.