The dissociation of the transthyretin (TTR) tetramer into a monomer is closely related to various TTR amyloidoses in humans. While the tetramer dissociation has been reported to be the rate-limiting step for TTR aggregation, few details are known about the mechanism. Here, molecular dynamics (MD) simulations were performed by combining conventional MD and biased metadynamics to investigate the mechanism for the wild-type (WT) and mutant (T119M) structures. Both were found to have a great deal in common. Conventional MD simulations reveal that interfacial hydrophobic interactions contribute significantly to stabilize the tetramer. Interfacial residues including L17, V20, L110, and V121 with close contacts form a hydrophobic channel. Metadynamics simulations indicate that the mouth opening of the hydrophobic channel is the first and the most difficult step for dissociation. Interactions of V20 between opposing dimers lock four monomers into the tetramer, and disruption of the interactions is found to be involved in the final step. During the dissociation, an increasing extent of solvation was observed by calculating the radial distribution functions of water around interfacial hydrophobic residues, suggesting that water plays a role in driving the tetramer dissociation. Moreover, compared to T119, residue M119 has a longer side chain that extends into the hydrophobic channel, making solvation more difficult, consistent with a higher energy barrier for dissociation of the T119M tetramer. This result provides a good explanation for the protective role of the T119M mutation. Overall, this study can provide atomic-level insights to better understand the pathogenesis of TTR amyloidosis and guide rational drug design in the future.