The aim of this study was to investigate the effects of nettle (Urtica dioica) hydroalcoholic extract (NHE) on saprolegniasis control, serum biochemistry, immunological and hematological indices of rainbow trout infected with Saprolegnia. Rainbow trout (n=180, weight=22±0.27g) were randomly distributed in six treatments. Experimental groups were first made prone to the disease by scaling the caudal peduncle, and then were exposed to bathing with S. zoospores (3×10⁵/L) for 4 hours. After infection, fish were stocked in 100 L tanks and received four treatments (50, 100, 150, and 200 mg/L of NHE) daily for 1 hour during 1 week. At the one-week bathing with NHE (first week), and a week after the bathing (second week), parameters of hematology [red blood cells (RBCs), white blood cells (WBCs), hematocrit, and hemoglobin], serum biochemistry (albumin, globulins, and total protein), immunity parameters (lysozyme activity, complement system, and respiratory burst activity), and mortality rate were examined in all treatments. Results showed that 100 mg/L of NHE significantly increased hematological, serum biochemical and immunity parameters in rainbow trout infected with saprolegnia (p< 0.05). Also, doses 50, 150 and 200 mg/L of NHE significantly reduced mortality rate with increasing fish resistance to saprolegniasis compared the control. Hence, the use of NHE at a dose of 100 mg/L with bath therapy is recommended to improve hematological and immunological parameters, and to control of saprolegniasis in this species.