Hypoeutectic Al–Ni alloys are characterised by the formation of as-cast structures composed by a dendritic aluminium-rich matrix surrounded by a eutectic mixture (α + β), where α is the Al-rich phase and β is the Al₃Ni intermetallics. The morphology, size and distribution of the intermetallic particles decisively affect the mechanical properties of these alloys. This study aims to determine experimental laws relating the primary dendritic arm spacing (λ₁) and Vickers microhardness (HV) of hypoeutectic Al–Ni alloys. Upward unidirectional solidification experiments were performed in order to obtain castings with significant differences in the scale of microstructural parameters along the casting length. It was found that the microhardness values were directly influenced by both the amount of solute and λ₁ and Hall–Petch-type equations relating the microindentation and λ₁ are proposed.