Undercover officers investigating reports of alcohol being served at a club after hours discovered the teenagers early Saturday. One teen approached an officer and made a deal to perform a sex act for money, police said.
"It's a very unusual circumstance," said police Sgt. Byron Fassett, supervisor of the high-risk victims and trafficking unit. "I haven't run into a case like this."
Both girls were taken into custody. Neither was being named because they are juveniles.
One of the teenagers is accused of compelling prostitution, a felony charge which involves causing "by any means a person younger than 17 years to commit prostitution." Her friend, also 13, is facing a prostitution charge.
Authorities declined to comment on how the girls got into the club.
"We believe there was an adult behind this," Fassett said. "We haven't arrested that adult, but 13-year-old girls can't get themselves to these locations."
Dallas police identified some 120 prostitutes in 2007 who were minors, most of them runaways.
City officials said they were outraged to hear about the case, as well as a December incident in which police say a 12-year-old girl was found dancing nude at a Dallas strip club.
Mayor Pro Tem Elba Garcia said Tuesday she would check whether the city can take action against such clubs.