"HIV/AIDS is returning to Thailand since the government awareness campaign started 20 years ago has fizzled out," said a bank spokesman who declined to be identified.
"We want the teenagers to be aware of the problem."
The health ministry has said it was concerned about the numbers of teenagers and homosexuals still being infected, though some significant decline in incidence has been claimed.
And despite Bangkok's reputation as one of the world's sex industry centers, Thailand is a generally conservative country.
The country's Disease Control Department chief Thawat Suntrajarn said embarrassment about buying condoms and ignorance in using them were the main causes of the new cases.
"Research papers from all sorts of agencies have a consensus that many condom users are embarrassed to buy condoms from counters," Thawat told a news agency.
"Women who buy condoms from convenience stores always get a strange look from people, so condom handouts are a good way to avoid such embarrassment."
New HIV/AIDS cases in Thailand, once praised by international health agencies for its aggressive campaign to tackle the epidemic, had fallen to 13,000 in 2006 from more than 100,000 a year in early 1990s, Thawat said.
But the worrying sign was that many of the new patients were teenagers and homosexual men, not heterosexual men in their 30s and prostitutes as in the past, he added.
A Health Ministry-commissioned survey last year showed 48 percent of 5,712 male high school students used condoms.
About 43 percent of 7,712 female high school students said their sex partners used condoms, it said.
Spurred by the findings, Thawat's department is running a television advertisement encouraging people to buy condoms despite criticism from conservatives who argue it encourages teenagers to be sexually active.
"Even those bank customers who don't need to use the condoms, they can pass them on to their families or friends," he said.