The main aim of the study was to investigate whether the Children’s Social Understanding Scale (CSUS), a parent report technique, is a valid and reliable measure of Theory of Mind (ToM) abilities in Polish-speaking children. Additionally, the role of language abilities in ToM development was explored. A group of 225 parents of approximately 3.5-year-old Polish children was tested using the Polish version of the CSUS. Their children were tested with a word comprehension test and five behavioural ToM tasks. Satisfactory psychometric properties of the Polish CSUS were observed and positive correlations between the CSUS and behavioural ToM tasks were found. A two-factor structure was found in the CSUS: Mental State Talk (MST) and Mental State Comprehension. The MST factor was significantly related to word comprehension. The utility of the Polish version of the CSUS for future cross-cultural research with this population is discussed.