The under-construction Winthrop Center project, set to become Boston's fourth-tallest building, has landed its first office tenant.

MP Boston, the local arm of development firm Millennium Partners, announced Thursday it signed investment firm Cambridge Associates to a 115K SF lease to move its global headquarters to the 691-foot-tall mixed-use tower in the Financial District.

Cambridge Associates plans to relocate in spring 2023 from its current headquarters at 125 High St., another Financial District building owned by Tishman Speyer. The new deal represents a downsizing from its previous footprint of 170K SF, the Boston Business Journal reported.

The 53-story Winthrop Center tower, designed by Handel Architects, will feature 812K SF of office space and 321 multifamily units. MP Boston said the project is on track to achieve the ambitious Passive House sustainability certification and will use 60% less energy than Boston's existing LEED Platinum buildings.

"Since Winthrop Center’s inception, it was important for us to find partners, like Cambridge Associates, who support our goal of delivering a healthy, energy efficient building that provides a solution to climate change,” Millennium Partners founder Christopher Jeffries said in a release.

Cambridge Associates CEO David Druley said in the release that the sustainability aspects were a key part of the firm's leasing decision.

"We will be able to meet our own significant commitments to sustainability and carbon reduction thanks to Winthrop Center’s approach to responsible, resourceful and renewable office space," Druley said.

The deal to fill a chunk of the building comes after the developer had to overcome significant obstacles brought on by the coronavirus pandemic. The project's construction financing deal fell through early in the pandemic after the developer had already dug the foundation, but MP Boston later landed a new $775M loan from an affiliate of London’s Cale Street Partners, the Boston Globe reported.

The deal is one of the largest Boston office leases signed since the start of the pandemic, and it continues the leasing momentum the city experienced late last year. The Boston office market recorded 1.5M SF of positive absorption in Q4, according to Colliers, its second consecutive quarter of occupancy gains after losing over 5M SF of occupancy earlier in the pandemic.