The potential effect of the element Ca on the precipitation behavior was investigated in an Mg-rare earth alloy. A combination of metastable β′′′ and β′ precipitates was observed for the peak aging condition at 200°C. Ca addition was found to have no significant effect on the precipitating phases and evolution sequence. Composition analysis showed that the Ca partitioned to both β′′′ and β′ precipitate phases. First-principles calculations indicated that Ca partitions to the rare-earth sublattice in the precipitate phase. This finding suggests the potential of Ca to partially replace costly rare-earth elements in precipitation-hardened Mg-rare earth alloys.