Reports from NBC News and CNN reported seeing the woman enter the federal courthouse in Orlando on Wednesday with her attorney.
The sighting could be a troubling development for the Northwest Florida Republican congressman that federal prosecutors are moving closer to indicting him.
NBC News reported that the ex-girlfriend, whose name was withheld to respect her privacy, has been in talks with prosecutors for an immunity deal in exchange for testifying about whether Gaetz had sex with a 17-year-old female for money in 2017.
Gaetz has not been charged with a crime and has repeatedly denied all accusations and called the federal investigation into him a "witch hunt."
"We have seen no credible basis for a charge against Congressman Gaetz," said Isabelle Kirshner, a New York-based attorney representing Gaetz, in a statement to the News Journal on Wednesday. "We remain steadfast in our commitment to challenge any allegations with the facts and law."
Citing legal sources familiar with the case, NBC News said the investigation into Gaetz is now focused on three crimes: sex trafficking the 17-year-old; violating the Mann Act, which prohibits taking women across state lines for prostitution and obstructing justice.
NBC News reported that the investigation into Gaetz stalled last year as prosecutors sought the cooperation of Gaetz's ex-girlfriend, whose testimony is crucial to the case, citing sources familiar with the investigation.
Gaetz's ex-girlfriend was in an open relationship with him in 2017 and 2018 and allegedly discussed other women he was involved with, NBC News reported.
Allegations that Gaetz paid for sex with a 17-year-old in 2017 became public last spring as Gaetz's former friend and Seminole County Tax Collector Joel Greenberg was indicted for sex trafficking.
Greenberg has since pleaded guilty to six of 33 charges against him including sex trafficking in a plea deal with prosecutors to testify against Gaetz in exchange for a lighter sentence.
Greenberg's sentencing has been delayed pending his cooperation in the case against Gaetz.