Twenty-first-century skills are increasingly recognized as critical skills that today's students must develop to adapt to increasingly rapid world changes. This research aims to test the effectiveness of the STEM-EDELCY learning model to improve students' 21st-century skills. This research involved 285 junior high school students (aged 12-13) from three state schools located in the city of Yogyakarta, Indonesia. This experimental research uses a three-factor pretest and posttest design. The instruments used were essay test questions and observation sheets. Data analysis was performed using ANCOVA and a paired sample t-test. The results of this research show a statistically significant influence between the use of learning models on students' 21st-century skills posttest scores. Post-hoc testing revealed that students in the STEM-EDELCY group had a higher level of 21st-century skills than students in the 5E-learning cycle group and the guided inquiry group. The STEM-EDELCY model is effective for developing 21st-century skills.