The present investigation is the first examination of the factor structures, reliability, external validity, longitudinal invariance, and stability of the De Jong Gierveld Loneliness Scale (DJGLS), as used with early adolescents. It is based on a two-wave, large, representative sample of Polish primary school pupils. The results demonstrate that the model most reflective of the factor structure of the DJGLS is the bifactor model, which assumes the occurrence of one, highly reliable, general factor (overall sense of loneliness) and two, relatively irrelevant, subfactors. Essential unidimensionality (the general factor accounting for three fourth of the common variance) suggest that the interpretation of the subfactors over and above the general factor is inappropriate. The longitudinal confirmatory factor analysis indicated that the bifactor structure of the DJGLS is invariant over time. Correlations with self-rated loneliness, sociometric acceptance/rejection, social self-efficacy, identification with class group, family structure, and gender provide support for the validity of the DJGLS. This implies that it could be used as a measure of loneliness in adolescence, which does not involve references to the school context, making it possible to conduct studies that go beyond school period and compare the intensity of the feeling of loneliness in that group with other age groups.