A formal agreement hasn't been reached with the online magazine, but the photo shoot is a "foregone conclusion," said Johnston's attorney, Rex Butler.
Johnston fathered a son with Bristol, the 18-year-old daughter of Sarah Palin, the former Republican vice presidential candidate who resigned as Alaska's governor in July.
Publication of the photographs could be a source of embarrassment for Palin, often mentioned as a possible 2012 presidential candidate. Her memoir, "Going Rogue," will be published next month and pre-sales already have made it a national best seller.
After Sen. John McCain chose Palin as his running mate, Johnston was thrust into the national spotlight early in the campaign after Palin abruptly announced her unwed daughter was pregnant and the couple would marry.
The couple broke up after the birth of their son, Tripp, in December. The relationship between the Palins and Johnston since then has often been strained, mostly over visitation issues.
Palin's representatives did not respond to a request for comment on the Playgirl job.
Johnston also has been marketing himself for a possible modeling or acting career, spending time in New York and Los Angeles. His publicist, Tank Jones, said Johnston is fielding offers for other media jobs, including a reality show.
Johnston's first TV commercial, hawking Wonderful brand pistachios, debuted this week.
In the short spot, Johnston walks toward the camera with Jones, an Anchorage private investigator who also is Johnston's bodyguard. As they approach the camera, fan voices call out to Johnston off-screen. As he snaps open a pistachio, a voiceover declares, "Now Levi Johnston does it with protection" _ a reference that Johnston has said in interviews that he and Bristol sometimes had unprotected sex.
No date has been set for the Playgirl photo shoot, but Butler expects the world will get a gander of the finished product by the end of the year.
Playgirl spokesman Vincent Stevens couldn't immediately be reached for comment. Butler said Playgirl approached Johnston about posing in the buff.
"We told him he would have to do what actors and actresses do. They get in the gym," Butler said. "He got in the gym."
Johnston has been working out with Marvin Jones, a former Mr. Alaska competitor and the brother of Tank Jones. The trainer has put Johnston on a low-carb, high-protein diet.
After a month of rigorous workouts, Johnston is showing impressive results, Marvin Jones said. But Johnston had a head start as a former high school hockey player.
"It's kind of scary, he has so much energy. He's working out before we work out," the trainer said. "When it's time for him to bare all, I think he'll be ready."
If the low-key Johnston has any pre-exposure jitters, none of his handlers have noticed.
"Levi is a man of few words, but once you know him, you know he's very confident," Marvin Jones said.
It wouldn't be the first show-it-all with political ties. Patti Davis, the daughter of former president Ronald Reagan, posed nude for Playboy magazine in 1994.