Mageean has won gold, silver and bronze medals at the European Championships.
After recovering from a serious ankle injury, she came third in 2016 and was a runner-up in 2022 before topping the podium in Rome for Ireland last year.
Mageean made her Olympic debut in Rio in 2016 and competed at the delayed 2021 Games in Tokyo, where her preparations were hampered by a calf issue.
She was set to race at the Paris Olympics last year as a reigning European champion but an Achilles injury ruled her out on the eve of her 1500m heat.
Mageean’s stellar career has also included a silver medal at the Commonwealth Games in 2022, fourth place at the World Championships in 2023, and Irish records in the 800m, 1,000m, 1500m and mile.
She was named the 2023 Northern Ireland Sports Personality of the Year after an impressive year, in which she broke national records and was fourth in a competitive World Championships final.
European Athletics posted on X: “We’re all behind you, Ciara.
“Everyone at European Athletics sends their best wishes to Ciara Mageean following her recent cancer diagnosis.”
Portaferry GAA, who Mageean used to represent in camogie, said she is “truly an inspiration to so many people”.
“We are all behind you in this fight,” the club said.
“You and your family as always have our full support every step of the way.
“We know you’ll be back better and strong. We send our love and best wishes to you Ciara.”