This paper examines the linguistic relativity principle (Whorf, 1956) by investigating the impact of grammatical gender on cognition in simultaneous bilinguals of three-gendered Ukrainian and Russian. It examines whether speakers of three-gendered languages show grammatical gender effects on categorisation, empirically addressing claims that such effects are insignificant due to the presence of the neuter gender (Sera et al., 2002). We conducted two experiments using a similarity-judgment paradigm while manipulating the presence of neuter gender stimuli (Phillips & Boroditsky, 2003). Experiment 1, including neuter gender, revealed no significant effects, compatible with earlier studies on three-gendered languages. Conversely, Experiment 2, excluding neuter gender stimuli, showed significant language effects. Bilingual participants rated pairs as more similar when grammatical genders in both languages were congruent with the biological sex of a character. Significant effects were also found for pairs with mismatching grammatical genders in Ukrainian and Russian. Participants with higher proficiency in Ukrainian rated pairs as more similar when the grammatical gender of a noun in Ukrainian was congruent with the character’s biological sex, and incongruent in Russian. Our findings thus provide the first empirical demonstration that the exclusion of neuter gender online induces grammatical gender effects in speakers of three-gendered languages.