Juveniles of the protandrous hermaphrodite gilthead sea bream (Sparus aurata) were fed from early life stages to completion of sex maturation with plant-based diets formulated by BioMar. Total fish meal (FM) was included at 25% in the control diet (D1) and at 5% in the other three diets (D2, D3, D4). Added oil was either fish oil (FO) (D1) or a blend of vegetable oils replacing 58% (D2) and 84% (D3, D4) of FO. A commercial butyrate preparation (BP-70® NOREL) was added to the D4 diet at 0.4%. All fish grew fast through a three-year ongrowing cycle with overall specific growth rates of 1.4%, 0.7% and 0.5% for fish harvest at 300 g, 1 kg and 1.5–1.7 kg, respectively. Overall feed efficiency decreased progressively as fish size increased from 0.99 in 300 g fish to 0.8–0.7 in 1–1.7 kg fish. At the last stage, a clear sexual dimorphism was found for body weight and hepatosomatic index when all sampled fish were considered as a whole. A sexual dimorphism was also found for sex steroids with a peak of estradiol in the females sampled in October–December, whereas the peak of 11-ketotestosterone was delayed in males to December. Plasma levels of testosterone were similar in both sexes. The two first components of principal component analysis (PCA) explained >90% of total variance of plasma levels of sex steroids. The displacement along X-axis clearly separated males and females, whereas the movement along Y-axis was related to sampling time. An androgenic effect in the steroid plasma profile of fish fed plant-based diets was also shown, which was especially evident for the low FM/FO diet formulation (D3). This effect was reversed by butyrate and the female/male ratio of D4 fish (age class +3) did not differ from that of control fish, whereas the proportion of females in D2 + D3 fish was higher (P < 0.06) than in control fish (64% vs. 50%). This group also showed increased circulating levels of vitellogenin, non-detectable in males/intersex fish of any experimental group. Taken together, well balanced plant-based diets did not compromise the maximum growth of farmed gilthead sea bream over a 3-year production cycle, and data on key performance indicators with a wide range of FM/FO formulations can be considered as reference values for the species. It is noteworthy that the plant-based diets had an impact on sex reversal, but this effect was mitigated by butyrate supplementation. This work has been carried out with financial support from the European Union (ARRAINA, FP7-KBBE-2011-5-288925, Advanced Research Initiatives for Nutrition and Aquaculture). Additional funding was obtained from the Spanish MINECO (MI2-Fish, AGL2013-48560; Pubertrait, AGL2016-75400) and from Generalitat Valenciana (PROMETEO FASE II-2014/085 and 051). Peer reviewed