The TV host, 57, was admitted to hospital in Majorca during a family holiday.
He posted a picture on Instagram of tubes in his arm and his hospital identity tags around his wrist, writing: “Not the sort of bangles I usually choose on holiday.”
He was on a break with his family after shooting the Amazon Prime show, when he fell ill. He had planned to return to work after his holiday.
In a message on Twitter, the ex-Top Gear said he was “in a wheelchair, connected up to tubes, in a hospital”.
A spokesman for The Grand Tour confirmed the presenter was being treated for pneumonia.