The Multicolour Size--Ramsey Number of an Even Cycle

We determine the $k$-colour size–Ramsey number of even cycles up to absolute constant factors. For every $k\ge 2$ and every even $n\ge 100\log k$, $${\widehat{R}}_k(C_n)=\mathrm{\Theta }(k^2\log k)n\mathrm{.}$$ The lower bound follows from the corresponding result of Beke, Li and Sahasrabudhe for paths, while our upper bound improves the previous best estimate $O(k^{34}n)$ of Javadi, Kohayakawa and Miralaei.