About 7,000 married people reported that they had HIV/AIDS in 2006, from a total of 17,000 new cases, according to the latest survey by the ministry's disease control department.
Forty percent of new infections in married couples were reported by wives who said they got the disease from their husbands, the survey said, while 10 percent of the cases were husbands who were infected by their wives.
Extra-marital affairs and men having sex with prostitutes accounted for the rest of the infections.
The survey also said that condom use among couples in 2005 was at a very low level of around 44 to 52 percent.
But overall new infections reported in 2006 were down from 18,000 in 2005.
"Despite the decrease of overall new infections, there are some signs that the AIDS problem in Thailand might get worse again," said Thawat Suntrajarn, the department's director-general.
He called the spike in infections among married couples "worrisome".
Thawat said the ministry hoped to see the number of infections this year drop to 14,000 -- just 10 percent of the 140,000 reported in 1987.
HIV/AIDS is one of Thailand's top causes of death, with some 500,000 Thais living with the disease, according to health ministry figures.
Thailand's universal HIV/AIDS treatment programme has been hailed as a success in the fight against the disease, largely because of the country's ability to provide antiretroviral drugs to patients.