Accurate determination of reservoir petrophysical parameters is important for formation evaluation and reservoir management. In this paper, we present a joint inversion approach for the estimation of reservoir petrophysical parameters, such as porosity and fluid saturations, from electromagnetic and seismic measurements. Electromagnetic data are linked to porosity and fluid saturations through Archie's equations, while seismic data are linked to porosity and fluid saturations through a rock physics fluid substitution model. The approach provides substantial advantage for an improved estimation of porosity and fluid saturation distribution over the one obtained from the separate inversion of electromagnetic and seismic data. Sensitivity studies combined with inversion tests show that seismic data are mainly sensitive to the porosity distribution, while electromagnetic data are sensitive to the fluid saturation distribution. The inversion of either electromagnetic or seismic data is not unique, and thus leading to ambiguity in the determination of porosity and fluid saturations. We show that the joint inversion approach reduces the ambiguity on the determination of porosity and fluid saturations, thus leading to improved reservoir characterization. The concept and the implemented inversion method are demonstrated using various cross-well survey examples.