Retired Major League Baseball star Ryan Howard has sold his custom-built Tampa Bay-area megamansion for $16.5 million in an off-market deal—setting a record for the most expensive sale in the Tampa Bay area, Mansion Global has learned.

It helped that the palatial waterfront home perched on the Gulf Coast is also one of the largest houses in the area. It spans more than 19,500 square feet—but more than 37,000 if you count the home’s numerous covered terraces and balconies. The cavernous interiors include eight bedrooms, seven bathrooms and four powder rooms.

The only sale in recent memory that comes close to Mr. Howard’s estate is a historic, 14-bedroom mansion in nearby Clearwater, Florida, that sold in 2017 for $11.18 million, according to sales records on Zillow.

Mr. Howard, 39, an All-Star first baseman with the Philadelphia Phillies, purchased the underlying land in the posh beachside area of Belleair Shores in 2011 for $3.5 million, according to property records, and spent the next three years building an Italian-inspired palazzo.

While it’s replete with luxury amenities and decorative flourishes, the most unorthodox feature is a manmade lazy river that flows through the ground floor, turning the underbelly of the home into a grotto, and links both sides of a massive freeform pool outside.

Mr. Howard has used the expansive ground-floor grotto as a lavish entertainment space, complete with a U-shaped bar, pool table and glass-enclosed bowling alley, said Jackie Diaz, who co-listed the property with Karen Hegemeier, both of Premier Sotheby’s International Realty’s Clearwater office.

Inside, the home features soaring cathedral ceilings and living rooms where walls of glass fold back and open out onto the balconies overlooking the Gulf of Mexico. The home has more than double the average water frontage for Belleair Shores, spanning roughly 200 feet of beach.

Mr. Howard went to great lengths to keep the whisper listing truly “hush-hush,” according to his agents, who devised a confidential marketing plan.

They even created a secret website accessible only to potential buyers and agents who’d signed a non-disclosure agreement ahead of viewing the home, Ms. Diaz explained.

“They were very private people,” Ms. Diaz said. Mr. Howard declined to comment for the story.

Now that the deal’s done, photos of the property reveal Mr. Howard kept numerous momentos of his 13-season career with the Phillies from 2004 to 2016. Framed jerseys decorate the bowling alley, as well as the dedicated trophy room.

The mansion also boasts a home theater, sauna, indoor games room and two-bedroom guest apartment.