China-born Andy Zhang will become tournament's youngest player since Second World War.
14-year-old given US Open chance after Paul Casey pulls out through injury
China-born Andy Zhang enters the field after Casey's ongoing injury problems ruled him out of contention for the competition, which starts on Thursday.
The previous youngest competitor in the post-war era was Tadd Fujikawa, who was 15 at the 2006 US Open at Winged Foot, according to the US Golf Association.
Casey has been beset by injuries this year, and missed the first two months of the season following a snowboarding accident at Christmas.
"You're always disappointed to miss time due to injury, but it's especially true when you have to withdraw from the US Open," the world number three admitted.
Zhang, who attends the IMG Golf Academy in Florida, narrowly missed a 33-foot eagle attempt on his last hole in the US Open qualifying, then lost a playoff to Brooks Koepka to settle for alternate status.
The teenager is scheduled to tee off at 1821 local time on Thursday from Olympic Club's Lake Course, alongside Hiroyuki Fujita of Japan and American Mark Wilson.
Brandt Snedeker also withdrew Monday because of injury, with amateur Jordan Spieth of Texas taking his place in the field.