The quantum phase of a multiparticle system has remained a long-standing mystery in quantum physics. The absence of a phase operator that faithfully represents this physical quantity has prevented the formulation of a meaningful uncertainty relation with particle number. Here, we resolve this problem by measuring, for the first time, the quantum phase of multiparticle systems containing up to fifteen particles. We achieve this through a new quantum state tomography scheme that combines homodyne interferometry with photon-number-resolving detection. This approach enables us to reconstruct complementary photon-number and phase representations and establish the corresponding number-phase uncertainty relation. Our measurements further provide access to the mutually unbiased basis of a multiparticle system, a fundamental resource for multiparticle quantum information processing. Furthermore, we report the spatially resolved characterization of the quantum phase. These results provide an operational framework for measuring quantum phase in multiparticle systems, with implications for fundamental quantum physics and scalable multiparticle quantum technologies.