All of the victims were older adults with additional underlying medical problems, according to the office of Mayor Bill de Blasio.
The office said Monday that there have been 81 reported cases of the flu-like disease. Sixty-four people have been hospitalized.
Legionnaires' disease is a respiratory bacterial infection usually spread through mist that comes from a water source, such as cooling towers, air conditioning or showers. It is not transmitted person to person. Symptoms of the disease include fever, chills and a cough.
Most people recover, but between 5% to 30% of those who get the disease die, according to the U.S. Centers for Disease Control and Prevention.
"We are conducting a swift investigation to determine the source of the outbreak and prevent future cases. I urge anyone with symptoms to seek medical attention right away," Dr. Mary Bassett, commissioner of the health department, said last week.
The department's probe includes testing water from potential sources in the area.
Five buildings in the outbreak cluster, including a hospital and a hotel, have so far tested positive for traces of the legionella bacteria. Environmental cleaning and taking any corrective measures needed has been completed in all five locations, the city said Sunday.
"The New York City water supply does not pose a risk, so people should continue to feel confident in drinking tap water to stay cool during this period of hot weather," said Bassett.
At a City Hall news conference Thursday afternoon, de Blasio warned that anyone with symptoms should get tested, but he said that most New Yorkers are not at risk.
The city Health Department continues to employ disease detectives to investigate the source of the outbreak and is conducting outreach to vulnerable populations including the elderly and homeless. A town hall discussion between community members and city medical experts entitled "The Facts About Legionnaires Disease" was scheduled for Monday evening.
Legionnaires' disease outbreak kills 2 at Chicago hotel
Bronx Borough President Ruben Diaz Jr. said on Friday that his office will push for legislation to create a citywide inspection system that would test building water infrastructure for the disease and others.
"The city must create a new inspection system for these systems, just as we inspect other critical systems such as elevators," Diaz said in statement.
An outbreak of the disease killed two people at a hotel in downtown Chicago in 2012.
VA under scrutiny after Legionnaires' cases in Pittsburgh
An estimated 8,000 to 18,000 hospitalized cases of Legionaires' disease occurs each year in the United States. An outbreak of the disease in Philadelphia in 1976, largely among people attending a convention of the American Legion, led to its name.
CNN's Doug Criss and Lauriel Cleveland contributed to this report.