Boston-based Samuels & Assoc. has landed an anchor tenant for its 237,000-square-foot Van Ness office tower at 1325 Boylston St. in the Fenway.

Digital health care company Optum will relocate 800 employees this spring into 125,000 square feet at the Van Ness, which was completed last year on speculation.

Samuels & Assoc. has been marketing the Van Ness office space to health care and tech tenants looking for lower rents than hot office markets such as Cambridge and the Seaport. Health care risk manager CRICO relocated from 101 Main St. in Cambridge in late 2015 after leasing 45,000 square feet.

The Van Ness is part of a $250 million mixed-use complex that includes 200,000 square feet of retail space and 172 luxury apartments.

Samuels is renovating the neighboring Landmark Center to replace anchor tenant Blue Cross Blue Shield. Samuels has converted part of the Landmark Center into a shared workspaces center called Hatch, which has attracted a group of startups, as well as office leases from Children’s Hospital and Harvard Pilgrim Health Care.

The vacancy rate in the 2 million-square-foot Fenway office market, including class A and B space, was 14.5 percent at year end, according to Colliers International’s Viewpoint report.

Optum, a division of St. Paul, Minnesota-based UnitedHealth, occupies office space at 10 Cabot Road in Medford, and 60,000 square feet at R.J. Kelly Co.’s 100 Quannapowit Parkway in Wakefield.