Agents in Brownsville working near the Rio Grande spotted three people, who were trying to enter the U.S. illegally, carrying the black duffel bag.
Authorities discovered the animal inside the bag as the alleged smugglers returned to Mexico.
The tiger was calm, possibly because it had been sedated. He is believed to be about three to four months old.
He's now being cared for at the Gladys Porter Zoo in Brownsville, Texas.