The bear, called Nissan, was in slow-moving traffic outside the French town when a group of people managed to open the back doors of his truck.
Three of them got into the compartment only to find the 22-month-old bear staring at them from his cage.
Nissan was on the final stages of his own journey from Eastern Europe in search of a new life in the UK - at the Yorkshire Wildlife Park in Doncaster.
He set off from Moscow Zoo earlier in the week by plane, but the second leg of his journey to South Yorkshire was by road from Frankfurt, Germany.
Simon Marsh, animal collections manager at the park, said the journey, which was masterminded by a specialist shipping firm, went completely to plan, apart from the issue in northern France.
He said: “There was a slight hiccup at Calais. Nissan had some unexpected guests in the back of the lorry.
“Obviously he was in a crate, so it was all very safe and secure, but I think they were a bit surprised when they saw a polar bear. It’s not probably what they were expecting when they opened the back of the lorry.”
Mr Marsh said: “Even at two years old he’s a big animal and he can be very dangerous.”
Asked what the danger would have been if Nissan had not been in his cage, he said: “I think a lot of people in Calais would have been in trouble if he hadn’t been in his crate.
“It’s obviously not ideal when people get in to the back of a lorry with a polar bear.”
Mr Marsh said Nissan is now getting used to life in Doncaster and staff are preparing to introduce him to the park’s two other polar bears, Victor, 16, and two-year-old Pixel. Victor is one of the biggest polar bears in Europe at 1200lbs.
Victor and Pixel were playing in the water on Wednesday at the 10 acre Project Polar reserve at the Yorkshire Wildlife Park.
Mr Marsh said they have shown an interest in the new arrival and the three bears, who are all male, will be put together some time in the next few weeks.
“Nissan is settling in really well and it is great to have him here,” said Mr Marsh.
He said the transfer was almost a year in planning with strict welfare regulations in force throughout the trip.
Nissan was born at Izhevsk Zoo, in Russia, on December 12, 2013 before later moving to Moscow. He has been moved to the UK as part of the European breeding programme.
The Yorkshire Wildlife Park plays a critical role in this by holding retired males like Victor or young males like Pixel and now Nissan, who are not currently needed in the breeding programme..
The purpose-built ten-acre site is divided into four sections, featuring landscaped hills, valleys and lakes with water up to eight metres deep.
Mr Marsh said: “Project Polar has been designed to replicate their Arctic tundra habitat and at 10 acres gives them the space to behave naturally. Males enjoy each other’s company and will spend time together in the wild so it is very natural to keep males together in captivity.
“We are happy now that we have Nissan here in great shape and I’m sure visitors will fall in love with him once he settles in and is ready to be out in the reserves and be seen by visitors.”
Simon Bridger, of West Malling, Kent, who witnessed the events in Calais, told The Sun: “They must have had the surprise of their lives. It shows how desperate they are to get to Britain.”