In recent years, the inefficacy and adverse effects of synthetic drugs and therapeutic agents used against the increasing prevalence of diseases in aquaculture have highlighted the need for introducing medicinal plants with antimicrobial properties into the sector. The use of plant-based products in the treatment of cultured fish species holds significant potential, particularly in supporting organic aquaculture practices. In this study, the antiparasitic activity of the ethanol extract of Artemisia absinthium L. (wormwood) was investigated against Hexamitiosis disease caused by Spironucleus salmonis in juvenile rainbow trout. The plant extract was incorporated into fish feed at doses of 1.5, 2.0, and 2.5 g/kg and administered to infected fish with an average body weight of 1.5-2.0 g for both short-term (7 days) and long-term (21 days) treatment periods. The results revealed that the A. absinthium extract exhibited antiparasitic efficacy against S. salmonis on both the 7th and 21st days. Cumulative mortality rates decreased in all experimental and positive control groups, with the 21-day application proving more effective than the 7-day treatment. The highest antiprotozoal activity was observed with the 2.5 g/kg dose on the 21st day, showing a statistically similar effect to that of the positive control (p<0.05). Based on these findings, it was concluded that ethanol extracts of A. absinthium could be used as a natural and alternative antiparasitic agent for controlling S. salmonis infections in rainbow trout aquaculture.