Planet Fitness has Revere and Salem locations, but North Shore Fit Vice President Taylor Mandell said Lynn is underserved when it comes to exercise places.

“All of our research shows it’s an untapped market and, as far as fitness facilities are concerned, it is underserved for a population its size,” Mandell said.

Johnnie’s closed in November 2012 and speculation over the 50 Boston St. building’s future use has surfaced and subsided up through Tuesday when Mandell confirmed Planet Fitness’ plans to pack more than 100 pieces of strength-building and cardiovascular equipment into 18,000 square feet inside the 28,000-square-foot building.

The rest of the building will be leased out to a yet-to-be-specified sublease tenant.

“We’re talking with three potential candidates,” Mandell said.

A Rite Aid franchise shares the building — owned by Newton-based New England Development.

Mandell, who is based in Portsmouth, N.H., said the Lynn Planet Fitness will also have tanning and hydro massage equipment as well as family fitness plans similar to those offered by the Revere and Salem franchises.

“It’s going to be packed with equipment,” Mandell said.

Lynn Planet Fitness will be open 24 hours a day, seven days a week with 15 employees. Mandell said North Shore Fit previously opened Planet Fitness gyms in Newburyport, Methuen, Billerica, Andover, Reading and Dracut.

“We’re a leader in low-cost, high-value gyms,” Mandell said.

He said interior demolition work will begin as soon as today. North Shore Fit will lease the former Johnnie’s site from New England Development.

Planet Fitness falls under “by-right” zoning status for the Johnnie’s building, meaning North Shore Fit only needs building permits to proceed with renovating the building and opening a gym. Ward 1 City Councilor Wayne Lozzi said he looks forward to welcoming Boston Street’s new addition.

“It’s great news — certainly given that we have had a vacant store there for a few years. I’m looking for a gym myself,” Lozzi said.

Johnnie’s rebirth as a gym excited Theresa Ferry, who lives about a block away from the Johnnie’s building and wants to find a place where she can work out.

“I’m all for it,” she said.

City Economic Development and Industrial Corporation Director James Cowdell told EDIC board members about Planet Fitness’ plans Tuesday night and briefed them on two other business openings, including R.F. O’Sullivan’s plan to open its downtown specialty burger pub the second week of February and potential plans for a restaurant in the Prime building on Washington Street, which Cowdell described as being “very much in the early stages.”

Lynn resident Kim Triantafyllou said she gets enough exercise working with preschoolers but said a Planet Fitness is good news for the city.

“Maybe it will get Lynn healthier,” she said.