This article studies the use of quality factor <inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> as a tool to design multiport antenna structures with matching networks. Approximative formulas for computing the <inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula>-factor from the port impedances of multiport antennas are presented. We show that the achievable performance level can be reliably estimated based on the <inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula>-factor without the need for computationally heavy and time-consuming matching network optimization. The efficiency of the proposed method is confirmed with a practical two-port antenna cluster design for smartphones operating in the 0.7–1.0 GHz frequency band. The results show that the proposed port impedance-based <inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula>-factor formulas provide an efficient tool to accelerate the design process of multiport antennas.