Several other people were found in the truck, which did not have a working air conditioning system despite blistering temperatures, and were taken to hospital.
The driver has been arrested but his identity was not released.
A person from the truck approached a Walmart employee in a car park and asked for water late on Saturday night or early Sunday morning, police in San Antonio confirmed.
Temperatures in the area reached a blistering 38C on Saturday.
The employee gave the person the water and then called police, and when officers arrived found the eight people dead in the back of the trailer, police Chief William McManus said.
Investigators checked store surveillance video, which showed vehicles had arrived and picked up other people from the tractor trailer.
"We're looking at a human trafficking crime this evening," Mr McManus said.
He called the case "a horrific tragedy".
The US Department of Homeland Security has also launched an investigation into the incident.
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