We investigate the Seebeck coefficient as a probe of collective transport behavior in graphene and quark-gluon plasma using a kinetic-theory approach in the hydrodynamic regime. The Seebeck coefficient is obtained by solving the Boltzmann transport equation in the relaxation-time approximation. At high-charge-carrier density, corresponding to the Fermi-liquid domain, our result approaches the behavior expected from the conventional Mott relation. In contrast, significant deviations from the Mott relation are observed in the low-carrier-density regime, corresponding to the Dirac fluid domain. These facts are in good agreement with experimental Seebeck coefficient data for graphene. This behavior indicates the emergence of collective hydrodynamic transport in graphene. The enthalpy per particle plays a key role for the Seebeck coefficient in the Dirac fluid regime. We extend our formalism to the ultra-relativistic quark-gluon plasma. A similar deviation from the Mott relation is observed, supporting a similar fluid response in the Seebeck coefficient across two markedly different strongly correlated quantum systems - graphene and quark matter.