The Port Authority has signed a 15-year lease to move its medical -services unit to 156 William St. in lower Manhattan.

The 8,459-square-foot lease covers part of the fourth floor in the 12-story building that has frontage on Ann and Beekman streets on the east side of the Financial District.

The 250,000-square-foot property is owned by the William Macklowe Co. and its joint venture partner, LaSalle Investment Management.

The PA’s medical-services offices are now located at 233 Park Avenue South and will move in spring 2017. The rest of the PA has already moved to the nearby 4 World Trade Center, and so the medical-unit move will be more convenient for employees.

Macklowe says it targeted this tenant and other medical facilities because the building is across the street from New York-Presbyterian/Lower Manhattan Hospital.

Paul Wexler and Josef Yadgarov of Wexler Healthcare Properties represented both sides in the deal that had an asking rent in the low $60s per square foot.

In the same building, legacy tenant Riverside Research Institute renewed its 17,000-square-foot lease for the entire ninth floor, represented by Richard McBride of Richard McBride Real Estate Services and Craig Reicher of CBRE. Wexler and Yadgarov repped ownership.

Weill Cornell Medical leased 85,000 square feet in the building, including an imaging center that is still under construction, Macklowe said.

Only two spaces of less than 4,000 square feet are available and will be pre-built for medical offices, Macklowe added.