This paper describes the sintering of an 18Ni(350) maraging steel with additions of boron, with the aim of producing high hardness rapid tooling. Reaction of the boron with the alloying elements in the maraging steel resulted in the formation of a Mo- and Ti-rich borides. The former melted at ∼1220 °C, providing a liquid phase for enhanced sintering. Although densification could occur regardless of the boron content, especially at high temperature, 0.4% B was required to produce a near full density component. The formation of the various borides depleted the matrix of critical age hardening elements. However, by altering the starting powder composition to compensate for this, hardness close to the wrought alloy has been achieved. This hardness was comparable to a common die casting tool steel. Examples of dies produced using selective laser sintering (SLS) are also shown.