"She put her arm around my waist as we took the picture but then she kept going lower and lower," the post read.
In addition, a woman named Victoria posted a Twitter thread alleging Sprouse had behaved aggressively with her after a party at his university dorm.
Reinhart, who plays Betty Cooper on Riverdale says "we are looking into legal action".
"This was incredibly triggering for me and scary because I've always supported survivors and believed them."
Sprouse posted on his Twitter account: "False accusations do tremendous damage to victims of actual assault.
"I encourage that people look into the accusations themselves, as the events detailed were factually untrue."
He added: "This seems to be the latest in a series of incidents seeking to baselessly cancel my castmates and me."
Madelaine Petsch, who plays Riverdale character Cheryl Blossom, joined her castmates in denouncing the claims, E! News reports.
"I am so taken aback right now," she said on her Instagram stories.
"My heart is really hurting for my friends and castmates who went through this today and I hope that everyone knows how serious it is to accuse somebody of that and I hope they understand the severity of what that does to people who have actually been through that."
Camila Mendes, who plays Veronica Lodge, posted a statement on her Instagram stories.
The statement said any false claims of sexual assault were "incredibly destructive" and called the allegations "sickening."
"Whatever point this person was trying to make about how quickly people believe what they read, it was not worth damaging the integrity of the Me Too movement."
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
