Role-play activities are considered a useful instructional design in enhancing the speaking performance of foreign language learners. However, in the traditional classroom context, learners may not readily have access to interlocutors for role-play activities. In this study, we proposed a designed method that integrated the GenAI agent into role-play activities and conducted a quasi-experiment at a Chinese university. A total of 53 Chinese students in an English course were assigned to an experimental group (n = 29) that engaged in role-playing with a GenAI agent and a control group (n = 24) that engaged in role-playing with their class peers. The results showed role-play activities with GenAI could facilitate students' speaking performance, and one-way ANCOVA showed there was no significant difference in speaking performance improvement when compared with traditional role-play activities with peers. Moreover, the paired-sample Wilcoxon test showed that the treatment students had significant intrinsic motivation and self-efficacy improvement, which were higher than the control group. The findings provide important implications for the design and implementation of GenAI-integrated role-play activities for foreign language speaking learning.