The present study adopted a sequential explanatory mixed-methods approach to explore the impact of e-tandem on English as a foreign language (EFL) learners' speaking skills (i.e. speaking fluency and coherence, lexicon, grammatical range and accuracy, and pronunciation) and willingness to communicate (WTC). To this end, two classes at a language institute were randomly assigned to an experimental group (e-tandem group) with 22 EFL learners and a control group (conventional group) with 20 EFL learners. The learners in the e-tandem class communicated with foreign English speakers online using Tandem, a language exchange application, and those in the conventional class had collaborative speaking activities during the class time. IELTS speaking skill test, willingness-to-communicate scale, and an individual semi-structured interview were used to collect the required quantitative and qualitative data. Paired samples t-test and one-way ANCOVA, used to analyse the quantitative data, indicated that e-tandem and conventional classes developed the EFL learners' speaking skills and WTC; however, the e-tandem group outperformed the conventional group on speaking skills and WTC. Thematic analysis, applied to analyse the qualitative data, generated a number of themes and categories signifying the e-tandem learners' positive attitudes and perceptions towards the e-tandem-based collaborative speaking activities. Pedagogical implications are discussed for collaborative speaking courses in EFL settings.