Grain boundaries in rare earth permanent magnets are of major importance to optimize coercivity and densification during sintering. In this work, grain boundary engineering of Nd-based ThMn12 magnets and their nitrides was systematically investigated. First, the parent compound with excess Nd (5, 20 and 40 at%) was studied. It is shown that a paramagnetic dhcp-Nd grain boundary is formed with excess of Nd above 20 at%. This intergranular phase promotes the liquid-phase sintering process at ∼ 700 °C and a bulk nanostructured Nd1.2(Fe,Mo)12 sample with a relative density of 95 % using SPS was obtained. By further doping with Cu, the grain boundary phase properties were tuned by lowering its melting point below 500 °C. Next, the focus is on the nitrides which have a coercivity of ∼ 0.6 T. During the nitrogenation process, the dhcp-Nd grain boundary phase reacts with nitrogen to form a paramagnetic NdN phase, which does not influence coercivity. However, NdN inhibits liquid-phase sintering of the ThMn12 magnets, due to its high melting point (> 1500 °C), limiting the relative density to 65 % after SPS. Thus, a proper grain boundary for nitrides is yet to be found but this work elucidates in detail the specific challenges of grain boundary engineering in Nd-based ThMn12 magnets, non-nitrided and nitrided. Display omitted •Tuning composition leads to a grain boundary allowing liquid sintering and magnetic decoupling.•The grain boundary (GB) can be tuned by adding Cu or Ga which reduces its melting point.•During nitrogenation, the dhcp-Nd GB phase reacts with nitrogen to form NdN phase.•The specific challenges of GB engineering in Nd-based ThMn12 magnets are elucidated.