The massive Baltimore penthouse of late novelist Tom Clancy is going on the market for $12 million, making it the highest-priced listing in the city, according to Shawn Breck of TTR Sotheby’s International Realty.

The approximately 12,000-square-foot, five-bedroom penthouse at the Ritz-Carlton Residences on Baltimore’s Inner Harbor is a combination of four apartments, Mr. Breck, one of the unit’s listing agents, said. With views of the harbor and downtown Baltimore, the home has six terraces, a gym and a home theater. One...