To develop an English for Specific Purposes (ESP) course for learners who would like to work as international relations officers (IROs), a Target Situation Analysis (TSA) is obligatory for course developers to arrive at a conclusion regarding the English language skills that learners have to master in order to work in this professional context successfully. The present study, therefore, aims to investigate which English language skills are required for working as an IRO. Overall, 400 Thai IROs were purposively selected to participate in the study. The data collection procedures were divided into the collection of quantitative data through a questionnaire and qualitative data through semi-structured interviews. The findings revealed that listening and reading skills were most required by IROs in various working contexts. These two receptive skills are generally utilized because IROs need to gather information to build knowledge for communicating with their foreign counterparts. However, speaking and writing skills are still widely applied by IROs, as they also need to deliver key messages precisely and appropriately. The results of this study offer ESP course developers valuable insights into the English skills that should be emphasized in ESP courses for IROs.