We have studied the magnetic and electrical properties of the polycrystalline ferromagnetic Ni50Mn35In15−xSix (1 ≤ x ≤ 5) Heusler alloys through magnetization, thermal expansion, and resistivity measurements. It was observed that an increase in Si concentration strongly affects the ground state of the martensitic phase and the magnetic properties of compounds. A magnetic phase diagram has been constructed for these alloys. It was found that both martensitic transition temperature (TM) and Curie temperature of austenitic phase (TC) decrease, while ferromagnetic ordering temperature of the martensitic phase increases with increasing Si concentration. The magnetoresistance (Δρ/ρ) associated with martensitic transformation was found to vary from –47% for x = 2 at T = 261 K to −26% for x = 5 at T = 230 K for a magnetic field change of 5 T.