The synthesis, crystal structure and magnetic properties of PrIn3−x Co x have been systematically investigated using X-ray diffraction and magnetic measurements. Co doping into PrIn3 causes the presence of satellite peaks, which has been indexed as a hexagonal structure. Rietveld’s structural refinement shows this structure is an isomorphic structure of CaIn2 type with a space group of P63/mmc and Z = 2. There are two non-equivalent crystal positions in one unit cell, which are occupied by 2Pr and 4(In, Co), respectively. Doping of Co into PrIn3 induces room temperature ferromagnetic order, and the magnetization increases with increasing Co content. It is suggested that the ferromagnetism in PrIn3−x Co x is caused by the spin–spin interactions between Co and Co or Co and Pr. The magnetic moment increase of PrIn3−x Co x could result from the partial substitution of non-magnetic element In by magnetic element Co in the host material through dilute doping of Co, and thus, a net moment is produced.