Gabby, who did not post her last name, shared a long statement alleging that she was assaulted by the actor who left her "sobbing and in pain".
The post contained distressing details of the encounter.
"Instead of asking me if I wanted to stop having sex knowing it was my first time and I was sobbing in pain and I didn't want to do it the only words that came out of his mouth were 'we need to break you in'," she claimed.
She went on to say that she "disassociated" herself and was in shock.
"I couldn't leave. I was only 5'2 and 98 pounds. He made me think this is how sex was supposed to be."
Gabby claimed that she told Elgort she was soon turning 17 when he shared his private Snapchat account with her. She also alleged that the actor shared nude photos of himself with her and shared screenshots of their messages to Twitter.
Gabby said she was coming forward "so I can finally heal" and said the experience has left her struggling with PTSD.
"I have panic attacks, I go to therapy."
The young woman also alleged the actor asked her for a threesome along with one of her dancer friends who was "as well underage". She claimed she didn't tell anyone because Elgort told her it could "ruin his career".
Representatives for Elgort did not respond when contacted by the Daily Mail for comment.
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youth services: (06) 3555 906
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• CASPER Suicide Prevention
If it is an emergency and you feel like you or someone else is at risk, call 111.
If you're in danger now:
• Phone the police on 111 or ask neighbours or friends to ring for you.
• Run outside and head for where there are other people.
• Scream for help so that your neighbours can hear you.
• Take the children with you.
• Don't stop to get anything else.
• If you are being abused, remember it's not your fault. Violence is never okay.
Where to go for help or more information:
• Women's Refuge: Free national crisis line operates 24/7 - 0800 refuge or 0800 733 843 www.womensrefuge.org.nz
• Shine, free national helpline 9am-11pm every day - 0508 744 633 www.2shine.org.nz
• It's Not Ok: Information line 0800 456 450 www.areyouok.org.nz
• Shakti: Providing specialist cultural services for African, Asian and Middle Eastern women and their children. Crisis line 24/7 0800 742 584
• Ministry of Justice: www.justice.govt.nz/family-justice/domestic-violence
• National Network of Stopping Violence: www.nnsvs.org.nz
• White Ribbon: Aiming to eliminate men's violence towards women, focusing this year on sexual violence and the issue of consent. http://www.whiteribbon.org.nz
