The charge against the 45-year-old rider was laid by the Marion County Sheriff's office on Wednesday. He was released by authorities on the same day on a $US2000 bond.
Jenifer Fisher, from the police department's public information office, told Horsetalk: "His partner came to us with some information he had found - some paperwork regarding some positive test result."
The partner alleged Chiacchia, a 2003 Pan American gold medalist, had initially told him he was not positive for any infection.
Ms Fisher said the partner recorded a conservation with Chiacchia in which he allegedly admitted that he knew he was HIV positive when they became sexually active.
The recording, she said, was not the result of any police surveillance. "The victim recorded it in his own time."
In Florida, is it illegal to knowingly have sex with another person while HIV positive without telling the other person beforehand.
Local media report that the victim told investigators that, between February and June 2009, he and Chiacchia had unprotected intercourse several times.
The victim had an email allegedly from Chiacchia in which the former Olympian admitted to knowing he was infected with the disease in 2008.
In March of 2008, Chiacchia was seriously injured on the cross-country of the Red Hills Horse Trials in Florida. His horse Baron Verdi hit a jump and somersaulted, falling on Chiacchia. He was unconscious for nearly two weeks and on a ventilator. A year later, he was back competing at advanced level and won at the Rocking Horse Spring Horse Trials riding his veteran campaigner, the trakehner stallion Windfall II.