Integrating humanities and arts into STEM has been suggested to better prepare students for the workforce. Studies have shown that improvisation (abbreviated as improv), an educational program from humanities and arts, can potentially improve engineering pedagogy and learning. However, little is known about improv's impact on developing undergraduate engineering students' growth mindset. Also, more work is needed to evaluate the impact of improv in improving the professional skills of undergraduate engineering students. This study integrated an improv workshop into a summer Research Experiences for Undergraduates (REU) program at a university in the east coast of the United States. A convergent mixed-method design was utilized to evaluate the impact of the integrated improv activity on the professional development of undergraduate engineering students during the REU program. The outcome of this study shows that the REU program, which included improv activities, significantly improved communication skills, students' growth mindset, and collaborative research skills. However, the REU program did not significantly improve the creativity ability of students. It is recommended that undergraduate research programs and engineering instructors incorporate improv activities into professional development and class time and consider implementing principles of improv when designing courses.