A $15 million donation to the Pine Street Inn in Boston could help create more than 400 housing units for people experiencing homelessness, leaders of the nonprofit announced Tuesday.

The gift from the Yawkey Foundation is the largest-ever made to the Boston organization, which operates emergency shelters and supportive housing in Greater Boston. Pine Street Inn Executive Director Lyndia Downie says the money will be used to expand both housing and connected services, including health care and job training. The new private effort to bolster services to the homeless comes as Boston officials grapple with how to clean up a tent encampment in an area known as Mass. and Cass, near Massachusetts Avenue and Melnea Cass Boulevard, that’s been an epicenter of the region’s opioid addiction crisis.

The Reverend Dr. Ray Hammond, a board member at the Yawkey Foundation, drew a line right to events at the troubled intersection and the lack of supportive services available.

“What we're seeing unfold tragically on Mass. and Cass is really an example of what can happen in the absence of those things: a job loss, an eviction or foreclosure, a bout with substance use disorder or an unanticipated medical expense,” he said.

Downie said the $15 million donation will bridge a gap in providing support services.

“There's a group of folks who need housing plus some degree of support so they can hold onto their housing,” she said. “There's no other alternative for them, which is why we've really put a stake in the ground and said, ‘supportive housing is the way for us to go.’”

Pine Street plans to increase the number of housing units it provides by 50% over the next five years.

The nonprofit supports about 2,000 men and women on a daily basis with permanent housing, emergency shelters, street outreach and employment training. Pine Street Inn is the largest homeless services provider in New England, with 850 tenants living in Boston and Brookline.

As a measure of its success, Downie pointed to a retention rate of 95% in Pine Street Inn's supportive housing.

“[It] means if you moved in a year ago, you were still there a year later,” she said. “And that's because the glue that the support services provides really make it possible for people to not just get housed, but to hold onto it and to really thrive.”