Since 2015, more than 560,000 South Sudanese primary school children have received psychosocial support (PSS) through the USAID-funded Integrated Essential Emergency Education Services program implemented by UNICEF. Several South Sudan-based nongovernmental organizations partnered with UNICEF to train local teachers to implement the PSS activities in child-friendly spaces. To evaluate the impact this intervention had on students' wellbeing and academic performance, a multi-institutional consortium of multidisciplinary partners purposively sampled 2,982 students and 580 teachers in 64 schools from five states in the Republic of South Sudan. Critical to the evaluation's aims was the design of a contextually relevant, rigorously validated instrument to measure students' wellbeing in a region where research on PSS outcomes in education in emergencies is needed. In this article, we first present the process by which these survey instruments were designed through the collaborative efforts of experts on measuring psychosocial support outcomes in conflict settings and experts on the local context. We then describe how we tested for the construct validity of the resulting instrument and present the results of our confirmatory factor analysis of its three-factor model of social wellbeing, emotional wellbeing, and resilience/coping. Finally, based on our process and the resulting instrument, we make recommendations for future research on PSS outcomes in emergency settings.