To get ready for his close-up, Johnston is training three hours a day, six nights a week at an Anchorage gym with a local body builder.
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.
Just after Sen. John McCain chose Palin as his running mate in August 2008, Johnston was thrust into the national spotlight when 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 first TV commercial, hawking Wonderful brand pistachios, debuted this week.
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 approached Johnston about posing in the buff, Butler said.
Playgirl spokesman Vincent Stevens couldn't immediately be reached for comment.
"We told him he would have to do what actors and actresses do," Butler said. "They get in the gym."
Johnston has been working out with Marvin Jones, a former Mr. Alaska competitor who has put the teenager on a low-carb, high-protein diet.