ABSTRACT The flipped instructional approach is gaining prominence in English as a foreign language (EFL) teaching. However, there is limited empirical evidence regarding its impact on students’ self-efficacy sources. This study aimed to investigate how EFL learners’ speaking self-efficacy sources – enactive mastery experience (EME), vicarious experience (VE), verbal persuasion (VP) and physiological and affective states (PAS) – change and function under the flipped instructional mode in comparison to the traditional mode. The study included 132 undergraduate students enrolled in two EFL-speaking classes at a Chinese university. The results of a multi-factor repeated ANOVA analysis revealed differences in the levels and changes of self-efficacy and self-efficacy sources between the two instructional modes, with the flipped classroom showing greater improvements. Hierarchical regression analysis provided support for the theoretical hypothesis regarding the contributions of the self-efficacy sources in the entire sample, with EME, VP, and PAS being significant contributors (VE was not). However, further analysis of the instructional modes indicated differences in the contributions. Qualitative data was also used to gain a more nuanced understanding of these relationships in the flipped instruction. At the end, this paper summarized the influence of the flipped instructional approach on EFL-speaking self-efficacy and its sources, and presents several pedagogical implications for foreign language instruction.