The involvement of relaxin (RLX) in nephritis has not been fully elucidated. In this study, we examined the expression of the RLX receptor and effects of RLX administration in antithymocyte serum (ATS) nephritis. The nephritic glomeruli showed an increased immunostaining for LGR7 compared to normal glomeruli. The administration of RLX for 7 days to rats with ATS nephritis did not cause an alteration in blood pressure or body weight. On the sixth day, however, a significant reduction in urinary protein was recognized in rats treated with RLX. Histological studies revealed the amelioration of glomerular accumulation of Periodic acid–Schiff stain–positive matrix in rats treated with RLX. Significant reductions of phosphorylated SMAD2, mesangial cell proliferation, and α-smooth muscle actin expression were observed in rats treated with RLX. These results suggest that RLX is a promising therapeutic tool for nephritic diseases.