David Witt, 57, the stepfather of Landis' wife, Amber, died on Tuesday of a self-inflicted gunshot wound to the head, San Diego Police said.
"The Landis family is devastated by the news of David's death," said Michael Henson, a spokesman for Floyd Landis. "They loved him dearly and they miss him. They hope that their privacy is respected in this time of mourning."
Police said Witt, of La Mesa, California, was discovered at 3:05 p.m. (11:05 p.m. British time) in a parking structure in the North Park section of San Diego. He was rushed to the hospital but died of the single gunshot wound about 7 hours after he was found.
Landis captured the prestigious Tour de France last month but ultimately tested positive for abnormally high levels of the male sex hormone testosterone.
The Murrieta, California, resident had produced one of the greatest solo rides in Tour history to win the 17th stage of cycling's biggest race after falling more than eight minutes behind during the previous stage.
Landis' team, Phonak, fired him after his second B sample tested positive for testosterone this month, and it appears likely he will become the first winner in the 103-year history of the Tour to be stripped of his title.
Witt, a San Diego restaurant owner with his wife Rose, was at the finish line in Paris when Landis won the race.
Police said they were unaware of any suicide note left by Witt.