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We consider the Steklov zeta function $\zeta$ $\Omega$ of a smooth boundedsimply connected planar domain $\Omega$ $\subset$ R 2 of perimeter 2$\pi$. Weprovide a first variation formula for $\zeta$ $\Omega$ under a smoothdeformation of the domain. On the base of the formula, we prove that, for everys $\in$ (--1, 0) $\cup$ (0, 1), the difference $\zeta$ $\Omega$ (s) -- 2$\zeta$R (s) is non-negative and is equal to zero if and only if $\Omega$ is a rounddisk ($\zeta$ R is the classical Riemann zeta function). Our approach givesalso an alternative proof of the inequality $\zeta$ $\Omega$ (s) -- 2$\zeta$ R(s) $\ge$ 0 for s $\in$ (--$\infty$, --1] $\cup$ (1, $\infty$); the latter factwas proved in our previous paper [2018] in a different way. We also provide analternative proof of the equality $\zeta$ $\Omega$ (0) = 2$\zeta$ R (0)obtained by Edward and Wu [1991].

An estimate for the Steklov zeta function of a planar domain derived from a first variation formula