The Related Group said Tuesday that it would acquire a four-acre site in Miami’s Brickell financial district that Related described as the “last undeveloped waterfront parcel of this magnitude.”

The Rivergate Plaza property at 444 Brickell Ave. and 77 SE Fifth St., to be purchased next month for an undisclosed sum, will be the site of a three-tower, mixed-use project being designed by the company.

The Capital Grille restaurant is a tenant in the building on the Miami River. Office tenants include the criminal appeals division of the state attorney general’s office and the law firm Bander & Associates.

The plan for the project west of the Brickell Bridge calls for more than 1,200 residential units, a hotel with more than 200 rooms and 200,000 square feet of office space and street-level retail, Related said in a statement. The development is expected to yield 2 million sellable square feet, Related said.

Miami-based Arquitectonica and Connecticut-based Pelli Clarke Pelli have been hired to design the towers, Related said.

“This is the most important development in the urban core,” Related chief executive Jorge Perez said in the statement.

The purchase of the site by Related had been rumored in the real estate community for weeks. The South Florida Business Journal reported last month the sale was being negotiated for close to $100 million. That would peg the price per square feet at around $575, which would be a record for the area.

Related already is developing several nearby projects, including the SLS Brickell condo-hotel development. Prices in the area have skyrocketed since the announcement of the mammoth Brickell CityCentre project.

In June, the company bought a smaller undeveloped site a few blocks south of 444 Brickell for $32 million.