This study experimentally investigates a series of phase transitions of ferromagnetic shape memory alloys, Ni2+xMnGa1−x, in the composition range 0.02 ≤ x ≤ 0.27. An experimental phase diagram of these alloys was constructed by the use of electrical resistivity, thermomagnetization, and thermoanalysis measurements, and compared with both the Ni-rich Ni2+xMn1−xGa and Mn-rich Ni2Mn1+yGa1−y alloy systems which have been extensively studied as proto-materials for ferromagnetic Heusler alloys. The comparison makes it clear that there are remarkable composition dependencies for the following characteristic temperatures. All three systems have the common property that Curie temperature decreases and martensitic transition temperature increases with increasing x till the both temperatures coincide each other. As for the magnetostructural coupling temperature, however, the Ni-rich system has a wide temperature range, and the Mn-rich system does not show signs of the magnetostructural coupling, whereas the magnetostructural coupling was observed in Ni2+xMnGa1−x alloys in the range 0.10 ≤ x ≤ 0.12 which has similar characteristics between the former two systems. The premartensitic transition temperature was also observed in Ni2+xMnGa1−x alloys; it reached the ambient temperature at x = ~0.04, the highest of the three alloy systems. In addition, an intermartensitic transition appeared in Ni2+xMnGa1−x alloys in the composition range 0.06 ≤ x ≤ 0.10, which showed a drastic change especially in the electrical resistivity. These characteristic temperatures are also compared in terms of the number of valence electrons among the Ni–Mn–Ga Heusler alloy systems, and the differences are clarified and discussed in detail.