The effects of international phonetic alphabet (IPA) instruction on English as a foreign language (EFL) adult learners' pronunciation have been well-recognized. However, not many studies on the topic were conducted in the Vietnamese context. Therefore, the current study aims to investigate (1) the impact of IPA learning on Vietnamese EFL adult learners' pronunciation and (2) adult learners' perceptions of the effects of learning the IPA system on their pronunciation. The study was designed as an experimental study, following a mixed-methods approach, using the pre- and post-tests, questionnaires, and interviews to collect quantitative and qualitative data. Thirty-eight adult learners took part in this investigation; they were divided into two groups, nineteen in the control and nineteen in the experimental group. The experimental study lasted ten weeks before the questionnaires and interviews were administered with the participants in the experimental group. The results demonstrated a significant improvement in adult learners' pronunciation in the experimental group. The participants in the experimental group also highly perceived the positive effects of learning the IPA system on their pronunciation. Pedagogical implications and suggestions were presented at the end of the paper.