It happened in the 12800 block of Mimosa Spring Drive in the Village Creek area some time after 3:30 p.m. Friday.
The Harris County Pct. 4 Constable's Office and Klein Volunteer Fire Department crews responded to the call for a 3-year-old that was found unresponsive.
EMS personnel performed CPR on the child as they rushed to a hospital.
The child had been in the car for three to four hours before he or she was discovered, authorities said.