Cottonwood Management Group, which recently began site work on a $800 million residential and retail development in Boston’s Seaport District, has leased 8,800 square feet in the nearby 101 Seaport office tower for its new regional office.

The Pasadena, California-based developer will build over 1 million square feet of apartments, condominiums and retail space in three towers on the 3.5-acre M block parcels as part of the 6.3-million-square-foot Seaport Square development. Cottonwood acquired the two parcels from Seaport Square master developer Boston Global Investors in October 2015 for $120 million.

Cottonwood was founded by MIT graduate Alexander Shing and earlier this year hired two local real estate executives, Edward Vydra from Taurus Investment Holdings and Mike Schumacher from Boston Properties to join Rebecca Mattson, head of East Coast development, in its Boston office.

The 101 Seaport offices also will be the future sales office for the M block development, which calls for 730 luxury condos and apartments scheduled for completion in 2019 and 2010.