The actress, who lost over four stone through healthy eating and exercise, is backing the Great Scottish Swim, which takes place on August 23. Swimmers will be able to take part in a half-mile, mile or two-mile event, with more than 20,000 people expected to participate across the UK.
Other locations for the events include Lake Windermere in Cumbria, the Royal Victoria Dock in London, Salford Quays in Manchester and Alton Water Reservoir near Ipswich.
Tyldesley said: "I am taking part in some of this year's events and although it will be a challenge, I enjoy using swimming as a fun way to keep fit and active."
Alex Jackson, event director of the Great Swim Series at organiser Nova International, said: "Catherine is a shining example of the benefits of living an active lifestyle. I am sure we will have new and returning participants from across Scotland and beyond feeling inspired to get into the water and take part in this iconic event."
Picture: Colin Mearns