The former Wales and British and Irish Lions captain is accused of "deceptively" spreading HIV to a previous partner.
Ian Baum claims that Mr Thomas hid his HIV status so that he wouldn't pass it on.
The TV personality is being sued for over one hundred thousand dollars.
The legal team for Mr Thomas has been asked to comment, but they are fighting the claim.
Mr Baum said in court papers that he was HIV negative when he had a relationship with him.
He is said to have discovered Mr Thomas had HIV when he saw a pill.
The documents said that the defendants were taking HIV medication.
Mr Baum made an appointment for a test. He was devastated when he found out he was positive.
According to the documents, Mr Thomas was very repentant and apologetic.
The relationship continued despite the defendants pleading with him to keep his HIV status secret.
Mr Baum claims that Mr Thomas used coercive and controlling behavior towards him in order to keep his HIV status a secret.
He claimed in the documents that the defendants caused him serious physical and psychological injuries.
He accused Mr Thomas of lying to him about his HIV status.
Mr Thomas, also known as Alfie, is accused of faking his HIV status in order to be a spokesman for HIV.
One year after publicly revealing he had the virus, Thomas was given a special prize for campaigning against HIV stigma.