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Simple Summary: The sustainability of aquaculture's growth is a worldwide challenge in the coming decades. Among the different approaches considered (e.g., implementation of recirculation systems, zero waste, etc.), reducing the use of raw materials imported from third countries might help to lower the carbon footprint of European aquaculture. Soybean meal (SBM) is one of the most widely used alternative raw materials to replace fishmeal, but is largely imported from Argentina, Brazil, and/or USA. Red vetchling (Lathyrus cicera) is a crop locally produced in Europe that might reduce the European dependency on SBM imports. Here, the replacement of SBM with red vetchling in diets for rainbow trout (Oncorhynchus mykiss) juveniles has been evaluated in different percentages (from 0 to 100%). Only fish fed a diet where SBM has been replaced at 100% showed lower growth, an altered amino acid profile in fish fillet, and some histopathological signs in the liver (greater % of pyknotic nuclei). Also, although temporally, glucose and triglycerides blood plasma levels were reduced, normal values were found at 24 h after feeding. Since vetchling meal can partially replace SBM without a negative effect on fish growth or physiology, its use might allow the reduction of SBM imports and the carbon footprint of European aquaculture. In fish diets, soybean meal (SBM) is still positioned as the most widely used alternative to replace fishmeal. Red vetchling (Lathyrus cicera), a crop locally produced in Europe, is here evaluated as a substitute for SBM. Rainbow trout (Oncorhynchus mykiss) juveniles (10.34 ± 0.04 g) were fed for 90 days. Six experimental diets replacing the SBM content at 0, 8, 16, 33, 66, and 100% (Named Control, T8, T16, T33, T66, and T100) were tested. Growth performance and fish fillet amino acid composition were only significantly reduced in fish fed the T100 diet. Histopathological analysis showed that no major alterations were observed in the intestine, while T100 fish had a higher density of pyknotic nuclei in the hepatocytes than the Control, but similar hepatocyte surface coverage. Finally, postprandial levels of glucose and triglycerides in blood plasma decreased when red vetchling content was increased, but was only not fully restored after 24 h in the case of glucose in T66 and T100 fish. According to these results, red vetchling meal can replace up to 66% of the SBM without a negative effect on fish growth or physiology, representing a good alternative raw material for reducing European aquaculture's dependency on SBM imports and the carbon footprint of aquafeeds. [ABSTRACT FROM AUTHOR]