We study dynamical phase transitions from antiferromagnetic to paramagnetic states driven by an interaction quench in the fermionic Hubbard model using the nonequilibrium dynamical mean-field theory. We identify two dynamical transition points where the relaxation behavior qualitatively changes: one corresponds to the thermal phase transition at which the order parameter decays critically slowly in a power law ∝ t −1/2 , and the other is connected to the existence of nonthermal antiferromagnetic order in systems with effective temperature above the thermal critical temperature. The frequency of the amplitude mode extrapolates to zero as one approaches the nonthermal (quasi)critical point, and thermalization is significantly delayed by the trapping in the nonthermal state. A slow relaxation of the nonthermal order is followed by a faster thermalization process. PACS numbers: 71.10.Fd, 64.60.Ht In many physical systems out of equilibrium, phase transitions occur as a real-time process of symmetry breaking or symmetry recovery. Examples for such "dynamical phase transitions" include the evolution of the Universe [1], liquid helium [2], and photoinduced phase transition in solids [3][4][5]. The macroscopic aspects are often described by the timedependent Ginzburg-Landau theory, where the order parameter is supposed to vary sufficiently slowly in time and space, so that the system can be considered to be locally close to thermal equilibrium. On the other hand, recent experimental developments of time-resolved measurement techniques in solids [6] and cold atoms [7] allow one to study dynamical phase transitions very far from equilibrium on the microscopic time scale of correlated quantum systems. In these cases, a "near-equilibrium" description might not be applicable. For instance, it has been recently suggested that superconductivity can be induced above the equilibrium critical temperature (T c ) by coherently exciting certain lattice vibrations, and that it lasts for a relatively long time (a few tens of ps) before thermalization occurs [5]. This observation is reminiscent of the prethermalization phenomenon [8][9][10][11], or the dynamics in the presence of a nonthermal fixed point in relativistic quantum field theories [12]. A fundamental question that we pose here is if the existence of such a nonthermal fixed point in correlated condensed matter systems allows symmetry broken states to survive above T c , and how it affects the dynamics.An important and still unresolved issue is how to characterize a nonequilibrium phase transition and its critical behavior for quantum systems [13,14]. Previous studies have in particular focused on the dynamics near quantum phase transitions in low dimensional systems (e. g., Refs. [15][16][17][18]). Higher dimensional systems are usually expected to show a thermal criticality out of equilibrium since quantum fluctuations are well suppressed. In this Letter, we study a dynamical phase transition for a simple microscopic model of correlated materials, namely the Hubbard mode...