Michael Dewayne Johnson, 29, was on death row for the 1995 killing of a convenience store clerk near Waco.
Early Thursday, he slashed his own throat and arm with a makeshift blade fashioned from a small piece of metal attached to a wooden stick, said Michelle Lyons, spokeswoman for the Texas Department of Criminal Justice in Huntsville.
Prison guards had been checking on Johnson's welfare every 15 minutes, as is customary, when they found him unresponsive in a pool of blood in his cell, Lyons said. He was taken to a hospital in nearby Livingston, where he was pronounced dead, Lyons said.
"At a 2:30 a.m. check, Mr. Johnson was talking to prison staff, and had just eaten breakfast," Lyons said. "He had made no indications that he was contemplating suicide, nor has a note or other explanation been found."
Johnson's execution had been scheduled for 6 p.m. Thursday. His last-minute appeal was still pending before the U.S. Supreme Court.
His was at least the seventh suicide on death row in Texas.
In a recent prison interview, Johnson had denied gunning down 27-year-old Jeff Wetterman, who helped pump gas at the family store off Interstate 35 near Waco. Johnson blamed his companion, David Vest, for the killing.
"I never even saw the dude," Johnson said. "(Vest) jumped back into the car and we took off. He hollered: 'Go! Go! Go!'"
Vest blamed the shooting on Johnson, took an eight-year prison term in a plea deal and testified against his friend. Vest is now free.
Johnson's attorney Greg White argued in his client's Supreme Court appeal that Vest had admitted to the shooting but that the confession was improperly suppressed, depriving Johnson of a fair trial.
Johnson would have been the 22nd Texas inmate executed this year. The state now has 390 people on death row.