One hundred-degree readings were recorded just before 2 p.m. Thursday at Newark Liberty International Airport and Atlantic City International Airport in Pomona.
Most other areas were in the upper 90s.
People are being urged to stay where it's cool, drink lots of water and check on elderly relatives and neighbors.
Hospitals reported treating numerous patients with heat-related illnesses. Among them was a 93-year-old man who was in critical condition at St. Joseph's Hospital in Paterson with heat stroke.
Newark and other cities opened cooling centers for the day.
Many schools in New Jersey also closed early for a second day.