The National Weather Service has extended an air quality health advisory in the city through Tuesday night.
Temperatures are expected to reach into the 90s, but with the humidity, it could feel like closer to 100 degrees.
Conditions are particularly rough in the subways, making things pretty uncomfortable on platforms.
New York City Transit President Andy Byford says the agency is working to keep all trains and air conditionining running to keep straphangers cool.
"Unlike many systems, we have air-conditioned cars. I've worked in many a system where the cars themselves aren't air conditioned," Byford said. "So the key to me is to keep the service moving so that people do go down into the station, wait on the station for a minimal amount of time and then board a nice, air-conditioned train."
If you don't have access to air conditioning, cooling centers will be open across the city Monday.
To find the cooling center closest to you, call 311 or visit nyc.gov/beattheheat.
Everyone is encouraged to drink extra water and avoid strenuous activity.
The city says elderly and people who suffer from chronic health conditions are more vulnerable to the heat.