The 14-item Compulsive Internet Use Scale (CIUS) is one of the most frequently internationally adapted psychometric instruments developed to assess generalized problematic Internet use. Multiple adaptations of this instrument have led to versions in different languages (e.g., Arabic and French), and different numbers of items (e.g., from 5 to 16 items instead of the original 14). However, to date, the CIUS has never been simultaneously compared and validated in several languages and different versions. Consequently, the present study tested the psychometric properties of four CIUS versions (i.e., CIUS-14, CIUS-9, CIUS-7, and CIUS-5) across eight languages (i.e., German, French, English, Finnish, Spanish, Italian, Polish, and Hungarian) to (a) examine their psychometric properties, and (b) test their measurement invariance. These analyses also identified the optimal versions of the CIUS. The data were collected via online surveys administered to 4,226 voluntary participants from 15 countries, aged at least 18 years, and recruited from academic environments. All brief versions of the CIUS in all eight languages were validated. Dimensional, configural, and metric invariance were established across all languages for the CIUS-5, CIUS-7, and CIUS-9, but the CIUS-5 and CIUS-7 were slightly more suitable because their model fitted the ordinal estimate better, while for cross-comparisons, the CIUS-9 was slightly better. The brief versions of the CIUS are therefore reliable and structurally stable instruments that can be used for cross-cultural research across adult populations.