Abstract Two-dimensional transition metals borides Ti _x B _x have excellent magnetic and electronic properties and great potential in metal-ion batteries and energy storage. The thermal management is important for the safety and stability in these applications. We investigated the lattice dynamical and thermal transport properties of bulk-TiB₂ and its two-dimensional (2D) counterparts based on density functional theory combined with solving phonon Boltzmann transport equation. The Poisson’s ratio of bulk-TiB₂ is positive while it changes to negative for monolayer TiB₂. We found that dimension reduction can cause the room-temperature in-plane lattice thermal conductivity decrease, which is opposite the trend of MoS₂, MoSe₂, WSe₂ and SnSe. Additionally, the room temperature thermal conductivity of mono-TiB₂ is only one sixth of that for bulk-TiB₂. It is attributed to the higher Debye temperature and stronger bonding stiffness in bulk-TiB₂. The bulk-TiB₂ has higher phonon group velocity and weaker anharmonic effect comparing with its 2D counterparts. On the other hand, the room temperature lattice thermal conductivity of mono-Ti₂B₂ is two times higher than that of mono-TiB₂, which is due to three-phonon selection rule caused by the horizontal mirror symmetry.