In this paper, we report a semianalytical model for the mass transfer impedance of a microfluidic electrochemical chip (MEC). The model is based on the molar advection--diffusion equation for a microfluidic channel with a Poiseuille flow and an electrochemical reaction at the interface of deposited electrodes. Fourier--Laplace integral transforms and the quadrupole formalism are used to obtain a solution to these equations, and the three-dimensional (3D) transient concentration and current density fields are computed. This solution is validated by in-operando concentration fields measured by a visible spectroscopic imaging technique, and several equivalent electrical circuits are proposed to model mass transfer in MECs. The proposed method is the fastest way to compute the 3D transient mass transfer impedance, which can be used for a large variety of applications, such as MEC-based cytometry measurements or to predict the current density in a fuel cell.