Attitudes towards open peer review, open data and use of preprints influence scientists’ engagement with those practices. Yet there is a relatively small number of validated questionnaires that measure these attitudes. The goal of our study was to construct and validate such a questionnaire. Using a sample of Croatian scientists (N=541), from a wide range of disciplines, we developed a questionnaire titled Attitudes towards Open data sharing, preprinting, and peer-review (ATOPP). The questionnaire has 21 item with a four-factor structure (attitudes towards: open data, preprint servers, open peer-review and open peer-review in small scientific communities). Based on the questionnaire, the attitudes of Croatian scientists towards these topics were generally neutral, with a median of 3.3 out of 5 of the total attitude score. Croatian scientist attitudes were lowest for open peer-review in small scientific communities (Md 2.0) and highest for open data (Md 3.9).