Tujuan penelitian ini untuk mengetahui dampak PjBL dengan pendekatan STEM terhadap keterampilan berpikir kritis dan self efficacy peserta didik. Penelitian ini termasuk dalam jenis quasi experiment dengan Nonequivalent Control Group Design, melibatkan dua subjek penelitian yaitu experiment class and control class. Populasi penelitian ini adalah peserta didik kelas X IPA SMA Gajah Mada Bandar Lampung. Sampel yang digunakan sebanyak 2 kelas, pengambilan sampel adalah cluster random sampling, pengumpulan data menggunakan test esai berpikir kritis dan angket skala likert. Analisis data penelitian menggunakan uji Anova. Hasil penelitian menunjukkan bahwa nilai rata-rata pretest dan postest kelas eksperiment sebesar 64,96 menjadi 84,24. Model PjBL pendekatan STEM meningkatkan kemampuan berpikir kritis dan self efficacy peserta didik yang ditandai adanya peningkatan skor rata-rata pre test dan pos testt pada tiap indikator yaitu 60,86 menjadi 81,22. Peningkatan pre test dan post test dengan N-gain sebesar 0,55 atau 31%. Nilai rata-rata angket Self Efficacy kelas eksperimen sebesar 135,04 atau 93% dangan kategori sangat tinggi. Maka disimpulkan bahwa terdapat pengaruh model PjBL – STEM terhadap peningkatan keterampilan berpikir kritis dan self efficacy peserta didik kelas X pada materi ekosistem. An analysis of students' critical thinking skills and self-efficacy: The influence of PjBL-STEM on ecosystem learning materials Abstract: This research aimed to determine the influence of the PjBL model with the STEM approach on students' critical thinking skills and self-efficacy. This research is a quasi-experimental research with a nonequivalent control group design. This research also involved two research subjects, namely the experimental class and the control class. The population of this research was the tenth-grade students of SMA Gajah Mada Bandar Lampung. The samples were determined using the cluster random sampling technique. In collecting the data, the researchers employed a critical thinking test and a Likert scale questionnaire. The research data were analyzed using the ANOVA test. The results showed that the experimental class’s average score on the pretest experienced an improvement, from 64.96 to 84.24. The PjBL model with the STEM approach improved students' critical thinking skills and self-efficacy. The improvement was marked by the average pre-test and post-test scores improvement for each indicator, from 60.86 to 81.22. The improvement of pretest and posttest scores determined by the N-gain test was 0.55 or 31%. Furthermore, the average self-efficacy score obtained by the experimental class was 135.04, or 93% within the excellent category. Therefore, it can be concluded that the PjBL model with the STEM approach influenced the tenth-grade students' critical thinking skills and self-efficacy on ecosystem learning materials.