Participants slid gingerly into the water, a bracing 2.5-degrees Celsius (36.5 degrees Fahrenheit), and set off toward the finish line in the southern German town of Neuburg an der Donau four kilometres (2.5 miles) downriver.
Many let out an exhilarated squeal as they made contact with the water.
"This is my 18th time and I love every minute of it," said 62-year-old metalworker Hans Hartmann, one of 1,760 participants from across Europe.
"There is nothing like the feeling of swimming past the Neuburg skyline and hearing the fans cheer you on. If you're in good shape and the suit fits then it's not even that cold."
Twenty "polar bears" dispensed with thermal wetsuits and went in wearing just swimming trunks or a swimsuit, for a 350-metre-long dip in the Danube. Rainbow-coloured clown wigs and tall black witches' hats topped off the look.
The idea of the Donauschwimmen, as the event here is known, was born in 1970 when the local volunteer lifeguard service wanted to test whether it was
prepared to handle winter rescue operations, spokesman Roland Sammet said.
More than 220 groups now participate including firefighters, soldiers, diving clubs and hobby winter swimming enthusiasts.
"It's pretty chilly for the first five minutes until your body gets acclimatised," said 42-year-old Ludwig Kettner, a forklift operator who was on his sixth Danube winter swim. "After that, the adrenaline and the carnival atmosphere carry you through."
Frederic Moess, a 24-year-old engineering student from the northern German city of Kiel, said he had been training all year in the Baltic Sea for the big day in Bavaria.
"On the day itself, I like to have a few sausages, a soft pretzel and a beer or two to warm me up before I go in," he said with a grin.