Background: Regular moderate and vigorous-intensity physical activity (PA) is very important for health promotion and prevention of diseases. Frequency and the amount of PA can be monitored with direct and indirect measurements; the former is more accurate than the latter, which is, on the other hand, easier to use and more cost-effective. One of the most commonly used indirect methods is a self-report International Physical Activity Questionnaire (IPAQ), which comes in short (SF) and long forms (LF). Despite its wide cross-cultural use, IPAQ has not yet been adapted for use in the Slovenian language. The main purposes of the study were to produce a reliable Slovenian version of IPAQ-LF for use in education. Methods: A back-translation method involving four translators and one native English-speaking reviewer was used to adapt an IPAQ-LF questionnaire from English into the Slovenian language. The ease of use and comprehensibility of Slovenian IPAQ-LF was first pilot-tested on four teachers, then the reliability of a questionnaire was tested and retested on a sample of sixty-eight preschool and primary school teachers. Results: Collected data were analysed with several statistical methods, and the results revealed a high degree of test-retest reliability and internal consistency for both groups of teachers. Preschool teachers reported higher levels of moderate (1138±265 min/week vs. 870±1074 min/week) and vigorous-intensity (446±430 min/week vs. 106±157 min/week) PA as well as walking (996±337 min/week vs. 690±1173 min/week) than primary school teachers. Conclusions: The study has proven that the Slovenian IPAQ-LF version is reliable for use in education.