The purpose of this study is to determine the effects of the scenario-based STEM project design process on pre-service science teachers' perceptions of 21st-century skills, competencies, integrative STEM teaching intentions, and STEM attitude. In the study, a pretest-posttest quasi-experimental design, which does not contain a control group, was used. The participants of the research are 66 fourth-grade pre-service science teachers in the south-west of Turkey. The 21st-century skills and competencies scale and an integrative STEM teaching intentions questionnaire, and a STEM attitudes scale were used as a pre-test and post-test. Pre-service teachers designed projects using engineering design processes to solve the problems in the scenarios given to them. In the analysis of the data, paired-samples t-test and Wilcoxon Signed Rank Test were applied using SPSS-21. The results show that designing a scenario-based STEM project certainly contributed to the 21st-century skills competence perceptions, STEM teaching intentions, and STEM attitudes of pre-service science teachers.