Many researchers argue that science, technology, engineering and math (STEM) instruction contribute to the national economy and educators attempt to develop integrated instructional programs. In the current study, the effect of a problem-based STEM activity on students' attitudes, career perceptions and career interests was investigated by using both qualitative and quantitative methods. The participants are 22 students attending a state secondary school in Turkey. Before and after the implementation of the problem-based learning (PBL) activity, the Student Attitudes toward STEM Scale was used to investigate the students' attitudes towards STEM and also the STEM Semantic Survey for Career Interest and STEM Career Interest Scale were applied to investigate their career interests. Following the application, a semi-structured interview form was used to determine students' opinions about problem-based STEM activities. The findings revealed that the students' attitudes towards STEM disciplines, STEM career interests in STEM-related occupations significantly increased. The students' interests particularly in the occupations related to engineering and technology were found to have considerably increased. The students' interview results also support these findings. During their interviews, the students stated that problem-based STEM instruction is effective in their learning and also it helps to develop their skills of the 21st century, makes the class more enjoyable, increases their interest in the profession of engineering and helps them to select their future career. As a result, integration of PBL with STEM positively affects students' attitudes and career perceptions that are in the pursuit of their future career.