In this letter we introduce a one-parameter deformation of two-dimensional quantum field theories generated by a non-analytic operator which we call Root-T T . For a conformal field theory, the operator coincides with the square-root of the T T operator. More generally, the operator is defined so that classically it is marginal and generates a flow which commutes with the T T -flow. Intriguingly, the Root-T T flow is closely related to the ModMax theory recently constructed by Bandos, Lechner, Sorokin and Townsend.