The crystal structures, magnetic and dielectric properties were evaluated for Sm1.5Sr0.5Ni1-xMnxO4 (x = 0.03, 0.05, 0.1) ceramics. The Rietveld results of X-ray diffraction patterns confirmed single orthorhombic phase with the space group of Bmab (64) were obtained in Sm1.5Sr0.5Ni1-xMnxO4 (x = 0.03, 0.05) ceramics, while in Sm1.5Sr0.5Ni0.9Mn0.1O4 ceramics, apart from the main orthorhombic phase, a tiny secondary phase was detected. A magnetism characteristic of a spin glass was found in Sm1.5Sr0.5NiO4, while a complicated magnetic phenomenon was observed in Sm1.5Sr0.5Ni0.9Mn0.1O4 ceramics. The giant dielectric response was observed in these ceramics, and dielectric loss decreased with increasing the content of manganese ions. After comparing the activation energies of dielectric relaxation and electrical conduction, the low-temperature giant dielectric response should be attributed to the adiabatic small polaronic hopping process, while at high-temperature, the low frequency relaxation was mainly attributed to the grain boundary effect. The suppression of dielectric loss should be benefited from the various polar region caused by Mn-substitution.