Let \(\varphi:[0,\infty)\to[0,\infty)\) be a strictly increasing, convex function with \(\varphi(0)=0\). The \textit{normalized Orlicz mixed volume} (regarding \(\varphi\)) of the convex bodies \(K,L\subset {\mathbb R}^n\) containing \(0\) in the interior is defined by \[ \overline{V}_\varphi(K,L)= \varphi^{-1}\left(\frac{1}{n|K|} \int_{S^{n-1}} \varphi\left(\frac{h_L}{h_K}\right)h_K\,dS_K\right). \] Here \(|\cdot|\) denotes the volume, \(S^{n-1}\) is the unit sphere, \(h_K\) is the support function and \(S_K\) is the surface area measure of \(K\). The optimization problem \[ \text{Maximize \(|E|\), subject to \(V_\varphi(K,E)\leq 1\), over all \(o\)-symmetric ellipsoids \(E\)} \] has, at it is shown here, a unique solution, which is denoted by \(E_\varphi K\) and is called the \textit{Orlicz-John ellipsoid} regarding \(\varphi\) of \(K\). This construction extends that of the \(L_p\) John ellipsoid, which was introduced by Lutwak, Yang and Zhang ([\textit{E. Lutwak} et al., Proc. Lond. Math. Soc. (3) 90, No. 2, 497--520 (2005; Zbl 1074.52005)]) and is obtained for \(\varphi(t)=t^p\) (\(1\leq p<\infty\)). This paper develops the theory of the Orlicz-John ellipsoid and proves, in particular, the following results. The Orlicz-John ellipsoid \(E_\varphi K\) is jointly continuous in \(\varphi\) and \(K\). As \(p\to \infty\), the Orlicz-John ellipsoid \(E_{\varphi^p}K\) converges to \(E_\infty K\), the origin-symmetric ellipsoid of maximal volume contained in \(K\). The volume inequalities \(|E_\infty K|\leq |E_\varphi K|\leq |E_1 K|\) and \(|E_\varphi K|\leq |K|\) hold, and for origin-symmetric \(K\), Ball's volume ratio inequality can immediately be extended via monotonicity, yielding that \(|K|/|E_\varphi K|\leq 2^n/\omega_n\) (where \(\omega_n\) is the volume of the \(n\)-dimensional unit ball); equality holds if and only if \(K\) is a parallelotope. Finally, the authors introduce an Orlicz surface area measure regarding \(\varphi\) and use its isotropicity to characterize the Orlicz-John ellipsoid.