Chiacchia, 45, did not appear in the Florida courtroom. The plea was entered on his behalf.
Chiacchia was arrested last month by the Marion County Sheriff's office over the alleged breach of a state law requiring any person to inform a sexual partner that they are HIV-positive.
A police spokesman told Horsetalk at the time: "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.
The partner recorded a conversation with Chiacchia in which he allegedly admitted that he knew he was HIV positive when they became sexually active.
The recording, the spokesman said, was not the result of any police surveillance. "The victim recorded it in his own time."
Local media reported at the time that the victim told investigators that, between February and June 2009, he and Chiacchia had unprotected intercourse several times.
The complainant allegedly had an email allegedly from Chiacchia in which the former Olympian admitted to knowing he was infected with the disease in 2008.
In March 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.
Chiacchia is a part-time Ocala resident and is on bail awaiting the next hearing.
Chiacchia won a bronze medal in equestrian team eventing in the 2004 Olympic Games.