This study investigated the trophic shift of young-of-the-year (YOY) thinlip grey mullet Liza ramada and golden grey mullet Liza aurata during their recruitment in a salt marsh located on the European Atlantic Ocean coast. Stable-isotope signatures (δ13C and δ15N) of the fishes followed a pattern, having enrichments in 13C and 15N with increasing fork length (LF): δ13C in fishes < 30 mm ranged from −19.5 to −15.0‰, whereas in fishes > 30 mm δ13C ranged from −15.8 to −12.7‰, closer to the level in salt-marsh food resources. Large differences between the δ15N values of mugilids and those of food sources (6·0‰ on average) showed that YOY are secondary consumers, similar to older individuals, when feeding in the salt marsh. YOY mugilids shift from browsing on pelagic prey to grazing on benthic resources from the salt marsh before reaching 30 mm LF. The results highlight the role of European salt marshes as nurseries for juvenile mugilids.