This paper introduces psychTestR, a new R package for designing and conducting behavioural psychological experiments. psychTestR allows users to combine data analysis and data collection into a coherent software pipeline, and therefore facilitates complex experiment designs where stimulus selection is guided by online analysis of existing response data. It supports a modular approach to experiment design, where code is parcelled up into easily distributable functions, modules, and packages. It provides a sophisticated internationalisation paradigm, whereby experiments are implemented with reference to multilingual dictionaries. It also provides powerful tools for automated testing, whereby the researcher can simulate a participant’s progression through the experiment and thereby test the robustness of the implementation. The resulting experiments can be deployed either to local computers for lab testing or to remote servers for online data collection. Software downloads, introductory articles, tutorials, and function-level documentation are available at the psychTestR website, http://psychtestr.com.