AbstractIn fetal alcohol spectrum disorders (FASD), brain growth deficiency is a hallmark of subjects both with fetal alcohol syndrome (FAS) and with non‐syndromic FASD (NS‐FASD, i.e., those without specific diagnostic features). However, although the cerebellum was suggested to be more severely undersized than the rest of the brain, it has not yet been given a specific place in the FASD diagnostic criteria where neuroanatomical features still count for little if anything in diagnostic specificity. We applied a combination of cerebellar segmentation tools on a 1.5 T 3DT1 brain MRI dataset from a monocentric population of 89 FASD (52 FAS, 37 NS‐FASD) and 126 typically developing controls (6–20 years old), providing 8 volumes: cerebellum, vermis and 3 lobes (anterior, posterior, inferior), plus total brain volume. After adjustment of confounders, the allometric scaling relationship between these cerebellar volumes (V_i) and the total brain or cerebellum volume (V_t) was fitted (V_i = bV_t^a), and the effect of group (FAS, control) on allometric scaling was evaluated. We then estimated for each cerebellar volume in the FAS population the deviation from the typical scaling (_vDTS) learned in the controls. Lastly, we trained and tested two classifiers to discriminate FAS from controls, one based on the total cerebellum _vDTS only, the other based on all the cerebellar _vDTS, comparing their performance both in the FAS and the NS‐FASD group. Allometric scaling was significantly different between FAS and control group for all the cerebellar volumes (p < .001). We confirmed the excess of total cerebellum volume deficit (_vDTS = −10.6%) and revealed an antero‐inferior‐posterior gradient of volumetric undersizing in the hemispheres (−12.4%, 1.1%, 2.0%, respectively) and the vermis (−16.7%, −9.2%, −8.6%, repectively). The classifier based on the intracerebellar gradient of _vDTS performed more efficiently than the one based on total cerebellum _vDTS only (AUC = 92% vs. 82%, p = .001). Setting a high probability threshold for >95% specificity of the classifiers, the gradient‐based classifier identified 35% of the NS‐FASD to have a FAS cerebellar phenotype, compared to 11% with the cerebellum‐only classifier (p_FISHER = 0.027). In a large series of FASD, this study details the volumetric undersizing within the cerebellum at the lobar and vermian level using allometric scaling, revealing an anterior‐inferior‐posterior gradient of vulnerability to prenatal alcohol exposure. It also strongly suggests that this intracerebellar gradient of volumetric undersizing may be a reliable neuroanatomical signature of FAS that could be used to improve the specificity of the diagnosis of NS‐FASD.