Whether speaking two or more languages (multilingualism) or dialects of one language (bidialectalism) affect executive function (EF) is controversial. Theoretically, these effects may depend on at least two conditions. First, the multilingual and bidialectal characteristics; particularly, (second) language proficiency and the sociolinguistic context of language use (e.g., Green & Abutalebi, 2013). Second, the EF aspects examined; specifically, recent accounts of the locus of the multilingual effect propose a general EF effect rather than an impact on specific processes (Bialystok, 2017). We compared 52 “monolingual” (with limited additional-language/dialect experience), 79 bidialectal, and 50 multilingual young adults in the diglossic context of Cyprus, where bidialectalism is widespread and Cypriot and Standard Greek are used in different everyday situations. Three EF processes were examined via seven tasks: inhibition, switching, working memory (Miyake et al., 2000). We found better multilingual and bidialectal performance in overall EF, an effect moderated by high (second) language proficiency.