The decision came after Armstrong announced he was dropping his fight against USADA's charges that he used performance-enhancing drugs to win the most prestigious event in the sport from 1999 to 2005.
Tygart told AFP that Armstrong would be stripped of all his results dating back to August 1, 1998.
He called the move a "sad day" for the sporting world but expressed hope that it would help create a "level playing field" for future athletes.
"It is a sad day for all of us who love sport and our athletic heroes," Tygart said in a separate written statement.
"This is a heartbreaking example of how the win-at-all-costs culture of sport, if left unchecked, will overtake fair, safe and honest competition, but for clean athletes, it is a reassuring reminder that there is hope for future generations to compete on a level playing field without the use of performance-enhancing drugs."
Tygart said USADA planned to hold a news conference on Friday to provide more details on the case.