Sherry Johnston will plead guilty July 31 in Palmer Superior Court to a count of possession with intent to deliver, her attorney, Rex Butler, said Friday.
In exchange, Butler said, the state has agreed to dismiss five other drug charges against his client, the mother of 19-year-old Levi Johnston.
Levi Johnston and Bristol Palin, the 18-year-old daughter of Alaska Gov. Sarah Palin, broke off their engagement after the Dec. 27 birth of their son, Tripp.
Sarah Palin was campaigning as the Republican vice presidential candidate last year when authorities began investigating Sherry Johnston, who later was accused of selling OxyContin to informants on three different occasions.
The case began in the second week of September when investigators intercepted a package containing 179 OxyContin pills. That led to the arrest of suspects, who agreed to be informants.
According to an affidavit, Johnston set up a meeting at a store. The document says the informant received $800 to make a purchase, meeting investigators later with 10 pills of 80-milligram OxyContin.
A second purchase was made the following day, according to authorities, and the informant wore a hidden camera and a microphone.
A third purchase occurred in late November, and the informant was wired for the transaction. Police videotaped the meeting.
Sherry Johnston was arrested in December after authorities served a search warrant at her home in Wasilla, about 40 miles north of Anchorage.
She has been out on bail since, and has traveled with Levi and her daughter, Mercede, on a national media tour, including interviews on "The Tyra Banks Show" and CNN's "Larry King Live."