Trailing 22-21 from the first leg of the Americas qualifying play-off in Santiago, the Chileans fought back to win the return 31-29 for a 52-51 aggregate triumph.
Chile, who lost 45-5 to Scotland A last month, trailed 19-0 after 30 minutes in Colorado but clawed their way back at Infinity Park.
The Chileans didn’t lead in the tie until there were five minutes left of the second leg. Even then, they received a late scare when referee Luke Pearce awarded the US a kickable penalty but reversed his decision due to a neck roll.
It is the first time Chile have qualified for the Rugby World Cup and they are the first new team in the tournament since Russia in 2011.