Emergency responders were called to an apartment complex in the 6500 block of Hollister Road near Wilshire Place Drive around 6:30 p.m.
At the scene, they found a 4-year-old girl suffering from heat exhaustion. She was taken to the hospital, where she was pronounced dead.
According to Houston police, earlier in the day, two women and several children, including the 4-year-old, were out running errands.
When they got home, everyone got out and unloaded the car, except the 4-year-old. Investigators said it appears she was forgotten.
Investigators believe the girl was left in the Chevy pickup for about an hour until a parent found her and called 911.
Lt. Larry Crowson offered advice to busy parents running errands, trying to juggle many things.
"Always take precaution. Try to remember where your kids are," Crowson said. "I know some people take precautions like leaving their phone or purse or keys to the house in the backseat with the children, so they don't forget to check on the children before they get out of the car."
According to the National Safety Council, four children died in hot cars in Texas last year. That's one of the highest numbers out of any state in the country, only behind Florida.
Experts say a child's body heats up faster than an adult's body. In just five minutes, a vehicle's temperature can rise up to 20 degrees.
Police have not said whether or not anyone will be charged in the little girl's death. The investigation is ongoing.
4-year-old dies after being left in vehicle in NW Houston, police say