<p>Diplomonad flagellates of the genus <i>Spironucleus</i> can cause mass mortalities and substantial economic losses in both ornamental and farmed fish species. The aim of the present study was to evaluate the antiparasitic activity of <i>Artemisia absinthium</i> L. essential oil nanoemulsions and ethanol extracts in the control of spironucleosis in rainbow trout (<i>Oncorhynchus mykiss</i>). Aerial parts of <i>A. absinthium</i> L. were obtained from local herbal markets and taxonomically identified by a plant specialist. A total of 1200 naturally infected rainbow trout (eight groups, three replicates, 50 fish per tank) were fed for 21&#xa0;days with diets containing essential oil nanoemulsions (1.0 and 3.0&#xa0;mL/kg) or ethanol extracts (1.0 and 3.0&#xa0;g/kg). The therapeutic effect was determined based on fish survival rate and parasite density in the intestine and feces. The chemical composition of <i>A. absinthium</i> L. was analyzed by GC–MS, revealing major constituents including sabinol (42.22%), chrysanthenylacetate (14.73%), epoxy-ocimene (4.97%), thujone (2.19%), d-isothujone (3.97%). The results showed that mortality rates were significantly (<i>p</i> &lt; 0.05) reduced in the nanoemulsion and extract groups compared to the negative control. The 3.0&#xa0;mL/kg essential oil nanoemulsion and 3&#xa0;g/kg ethanol extract groups showed significantly (<i>p</i> &lt; 0.05) better antiparasitic activity. These findings indicate that both essential oil nanoemulsion and ethanol extract forms of <i>A. absinthium</i> L. can be used in the control of spironucleosis infections in rainbow trout farms.</p>