  In any characteristic different from 2 and 5, Kond\=o gave an example of a K3
surface with a purely non-symplectic automorphism of order 50. The surface was
explicitly given as a double plane branched along a smooth sextic curve. In
this note we show that, in any characteristic $p\neq 2, 5$, a K3 surface with a
cyclic action of order 50 is isomorphic to the example of Kond\=o.
