We consider the zeta function Ω for the Dirichlet-to-Neumann operator of a simply connected planar domain Ωbounded by a smooth closed curve. We prove that, for a fixed real s satisfying jsj > 1 and fixed length L.@ Ω/ of the boundary curve, the zeta function Ω.s/ reaches its unique minimum when Ωis a disk. This result is obtained by studying the difference Ω(s)-2L.@ Ω/ 2 π R.s/,where R stands for the classicalRiemann zeta function. The difference turns out to be non-negative for real s satisfying jsj > 1. We prove some growth properties of the difference as s →±∞ Two analogs of these results are also provided.