Trump had threatened to talk about Bill Clinton's past infidelities during the debate and has stepped up attacks on the former president since Friday.
"These four very courageous women have asked to be here," Trump said at the start of the meeting in St. Louis, host to the second of three planned debates before the Nov. 8 election.
Trump had threatened to talk about Bill Clinton's past infidelities during the debate and has stepped up attacks on the former president since Friday when a 2005 video emerged of Trump making vulgar comments about women.
Clinton's campaign responded to Trump's event by calling it a "stunt" and a "destructive race to the bottom."
"Hillary Clinton understands the opportunity in this town hall (debate) is to talk to voters on stage and in the audience about the issues that matter to them, and this stunt doesn’t change that," spokeswoman Jennifer Palmieri said.
None of the accusations was new. Bill Clinton was never charged in any of the cases, and he settled a sexual harassment suit with one of the women, Paula Jones, for $850,000 with no apology or admission of guilt.
While Clinton was president in 1998, the Republican-led House of Representative voted to approve articles of impeachment accusing him of perjury for misleading a grand jury about the nature of his relationship with former White House intern Monica Lewinsky.
In the 2005 video of Trump, the businessman, then a reality TV star, is heard talking on an open microphone about groping women and trying to seduce a married woman. The video was taped only months after Trump married his third wife, Melania.