The popular 37-year-old from Sheffield appeared to have been struck in the head by debris from Sage Karam's car when it crashed in front of him during the final laps on the “Tricky Triangle” of Pocono Raceway during the penultimate race of the season.
Wilson was rushed to a hospital by air ambulance.
“Wilson is currently in a coma and in critical condition while undergoing further evaluation at Lehigh Valley Health Network Cedar Crest Hospital in Allentown, Pennsylvania,” IndyCar said in a brief statement.
“IndyCar sends its thoughts and prayers to Justin, his family and Andretti Autosport during this difficult time.”
Karam was leading when his Chip Ganassi Racing Chevrolet spun on lap 179 of the 200-lap race. He was able to exit his car and was treated for a foot injury.
However, a piece of Karam's car, possibly the nose cone, appeared to strike Wilson, whose car then swerved into a wall.
Wilson's race-winning Andretti Autosport team-mate Ryan Hunter-Reay said he had been told Wilson was “unconscious and not responding.”
BAD LUCK DEAL
“With heads exposed and when you get flying debris it's a bad-luck deal,” Hunter-Reay said.
“Our thoughts are with him. I know him and his family well, they're great friends of ours. Really, really just thinking about Justin right now.”
“It's just a tough one right now,” said team owner Michael Andretti. “Our thoughts and prayers are with Justin right now. We're going to see. Hopefully, he's OK.”
Wilson has driven in major US open-wheel racing since the 2004 season.
He was driving a partial schedule in 2015 for Andretti Autosport, making his 174th IndyCar start.
He has seven wins in CART and IndyCar races, his most recent coming at Texas in 2012.
A married father of two daughters, Wilson lives in Colorado with his wife, Julia.
His brother Stefan Wilson, also a racing driver, tweeted that both he and Julia Wilson would travel to Pennsylvania to be with him.
There was an outpouring of support from Wilson's fellow drivers.
“It's always hard to see your competitors and your friends get hurt, so my prayers along with everyone else's prayers here are with Justin Wilson and his family,” said Colombian racer Gabby Chaves.
Japan's Takuma Sato added: “I heard Justin had a big accident and I hope he is OK. I don't know the status right now but I wish him a super-speedy recovery.”
It's not the first time that an IndyCar driver has been injured when hit in the head by debris in recent years.
James Hinchcliffe suffered a concussion when he was struck by flying parts at the 2014 Grand Prix of Indianapolis.
Original source: IndyCar driver fighting for life