This study was carried out within the scope of a nature education and science camp project. It aimed to investigate the effects of STEM activities conducted in out-of-school environments on teachers' STEM awareness and 21st-century skills. The research was designed according to the explanatory sequential pattern of the mixed method, and the quantitative and qualitative methods are the continuation of each other. Twenty-three science, elementary math, technology, and design teachers who worked at public middle schools participated in a six-day program that integrated STEM subjects with out-of-school learning. Seven different STEM-integrated out-of-school activities were implemented for six days in one of the central Anatolian regions in Turkey. STEM awareness and 21st-century skills scales were implemented in the study for data collection. A semi-structured interview was also conducted with teachers. The aim was for the teachers who participated in the project to integrate out-of-school and STEM education and then transfer this newly gained knowledge to their students. Both quantitative and qualitative findings support that teacher's acquisition of knowledge and skills throughout the science camp would support their students' craft, analytical thinking, motivation, problem-solving skills, and self-esteem. As such, out-of-learning environments should be used more in both STEM and 21st-century skills education.