The number of homeless people counted on the streets and in the shelters of the nation’s capital has dropped for the third straight year, D.C. officials announced Wednesday.

There were 6,521 homeless people in the District, a 5.5 percent decline from last year, according to the annual count performed by the city. The trend was driven by a 15.6 percent reduction in family homelessness, while the number of homeless, single adults increased by 2.8 percent.

The results were welcomed by the administration of Mayor Muriel E. Bowser (D), whose prom­ises to reduce homelessness — especially among families and children — were a core part of her first mayoral campaign. Since 2015, Bowser’s first year in office, family homelessness has dropped by nearly 24 percent, while overall homelessness has fallen by nearly 12 percent.

“We think this is a really important indicator of our progress,” said Laura Zeilinger, director of the D.C. Department of Human Services. “The system has been broken for many, many years, and we can see that we are turning it around in really important and meaningful ways.”

The count is based on a tabulation of those living on the streets — performed by city officials and volunteers — on a single night in January, as well as a census of families in the shelter system.

Homelessness also fell in neighboring Montgomery County, county officials said Wednesday, with 647 homeless people counted this year — a 23 percent drop from last year. It was the fourth year in a row that the county’s homeless population has shrunk.

In Fairfax County and the city of Falls Church, which perform a combined count, the number of homeless increased by 4.8 percent, to 1,034 people. Officials at other local governments in the region said they plan to release their count results later this week.

In the District, Zeilinger attributed the rise in homeless individuals to an ongoing shortage of affordable housing. While the city has invested heavily in recent years in building or preserving housing for those at the bottom of the income scale, the rise in rent and mortgage costs continues to outpace increases in wages, Zeilinger said.

“It comes down to economics,” she said. 

The increase in homeless single adults came even though 1,200 people in that category were moved into housing last year — indicating that many new people became homeless over that period.

Zeilinger said the number of homeless families was being driven downward by city policies, including the creation of smaller, more modern shelters — replacing the notoriously dilapidated family shelter that existed until last year at the former site of D.C. General Hospital — and front-end counseling efforts that help prevent families from entering shelters in the first place.

Some advocates of homeless people have criticized the latter initiatives, saying the city is sometimes refusing to offer shelter to women and children who need it, forcing them to couch-surf or stay in unstable living situations.

City officials say they will step up efforts to improve shelters and homeless services for single adults. The mayor has proposed setting aside $30 million in next year’s budget to replace the Harriet Tubman Women’s Shelter, in addition to funding for replacement of the 801 East Men’s Shelter. City officials have not yet released a timeline for when the replacement shelters would open.

“There is more work to do, and with new investments in our homeless services system, especially in our programs for single adults, we are going to continue getting more residents into safe, stable and permanent housing,” Bowser said in a statement.

Jennifer Barrios and Antonio Olivo contributed to this report.