Controlling the grain size and morphology of cast cobalt-based components is important for optimizing a component’s in-service properties. This work investigates the role of boron on the grain size of binary cobalt–boron alloys by application of contemporary grain refinement theory. Boron solute is found to refine the width of the columnar grains but fails to promote the columnar to equiaxed transition. The lack of equiaxed grains is attributed to the thermal solidification conditions and a lack of potent nucleant particles. The refinement of the columnar grains with boron solute may be due to a growth restriction mechanism.