The two teens are not related, police spokeswoman Margo Rohrbacher said.
Tia Trashae Powell was arrested on suspicion of pandering at about 4:30 p.m. on Canal Street after a patrol officer contacted the runaway girl, who was suspected of loitering for prostitution, Rohrbacher said.
The officer saw Powell walking by several times and discovered that Powell and the girl had been exchanging text messages about prostitution on their cell phones, Rohrbacher said.
"It was obvious the 18-year-old was the girl's pimp," Rohrbacher said.
The girl, also from Richmond, was booked into juvenile hall on suspicion of loitering for prostitution and being a runaway, and Powell was booked into Marin County Jail on a pandering charge, Rohrbacher said.
Powell's bail is $30,000.
Rohrbacher said five johns were arrested during an unrelated two-day prostitution sting earlier that week that involved two undercover female police officers in the Canal Street area.
Rohrbacher said the case is unusual because of the ages of Powell and the girl and because the alleged pimp is a woman.