Speaking English skill is crucial and advantageous for Vietnamese undergraduate students. Speaking English accurately and fluently, however, has proven difficult for these students due to limited English speaking environments, resulting in their insufficient language proficiency and unwillingness to speak. While artificial intelligence (AI) technologies are recommended as effective tools in foreign language education for their flexibility, interactivity, and learner-centeredness, enhancing students’ oral communication, little research has considered using an AI voice chatbot to improve students’ English speaking skills in Vietnam. Therefore, there are still gaps for this study to be conducted. An 8-week quasi-experiment was conducted with 30 Vietnamese undergraduate students. The participants were informed of research purposes, implementation, and confidentiality. They practiced English speaking with an AI voice chatbot in two class sessions per week, completed a pre-experiment and post-experiment speaking test, responded to a questionnaire, and participated in a semi-structured interview at the end. The results revealed students’ significant improvement in English speaking skills after using an AI voice chatbot to practice speaking with p < 0.05. The students also agreed that their English speaking skills improved after the intervention because they could speak English better by using suitable hedging words, grammar structures, and vocabulary. The findings pose positive ways for teachers to integrate the AI voice chatbot into their lesson plans for teaching and learning activities. Some limitations were found related to sample size and technological issues. The study brings about a new English learning environment to the Vietnamese EFL undergraduate.