Heather Unruh said her straight son suffered "tremendous anger and anxiety" after the surprise attack last year and went to police last week to fill out a report and hand over undisclosed "evidence."
She said Spacey (58) had stuck his hand down the 18-year-old's trousers and grabbed his genitals.
"A criminal investigation has begun," Unruh said, calling Spacey a "sexual predator."
"I want to see Kevin Spacey go to jail. I want to see Kevin Spacey have the hand of justice come down on him, not just for my son, but for the many others who have yet to speak their truth."
Unruh said that after the alleged assault, Spacey pressed her stunned son to go to a private after-party with him.
She said an unidentified woman noticed that her son was in distress, approached the teen and urged him to run, which he did.
Unruh spoke to Nantucket's sheriff shortly after the attack, but her son was not emotionally ready at the time to publicly accuse Spacey of sexual assault, she said.
"Nothing could have prepared my son for how that sexual assault would make him feel," she said. "It harmed him, and it cannot be undone."
Unruh said her son was "scared," and that it wasn't until the recent wave of allegations against movie producer Harvey Weinstein and other Hollywood heavyweights that she posted a cryptic Twitter message accusing Spacey of misconduct.
When Star Trek: Discovery actor Anthony Rapp accused Spacey of making inappropriate sexual advances toward him when he was just 14 years old, Unruh's son, now at college, became "committed to stopping Kevin Spacey," she said.
Speaking at a press conference with her lawyer Mitchell Garabedian on Wednesday, Unruh slammed Spacey for coming out as gay in his recent apology to Rapp (46).
"Shame on you for what you did to my son, and shame on you for using your apology to Anthony Rapp to come out as a gay man.
"That was an appalling attempt to deflect attention away from what you really are - a sexual predator. Your actions are criminal," she said.
Representatives for Spacey did not immediately respond to a request for comment.
The downfall for the two-time Academy Award-winning actor, once one of the most lauded of his generation, has been swift. He had been the lead in the now-canceled Netflix hit House of Cards.
Netflix's decision to finally cut ties with Spacey came after a CNN report last week, alleging that he sexually assaulted one production assistant on the set in Baltimore, Maryland, and created a "toxic" work environment through a pattern of sexual harassment that targeted young male crew members.
Spacey also was dropped by Creative Arts Agency, and British police have begun an investigation in response to an allegation that he sexually assaulted a male actor in London.
Spacey was reportedly seeking treatment at an undisclosed location.
