This research aims to investigate the effect of socio-scientific issues (SSI) based instruction on preservice science teachers' soft skills and environmental awareness. By applying cluster random sampling, 83 preservice science teachers (PSTs) were selected from the Department of Science Education at a university in Indonesia. The participants were divided into two groups which comprised the experimental group who were taught using SSI-based instruction, while the control group received direct instruction. The quasi-experimental study was carried out through a pre-and posttest control group design. Data was gathered quantitatively using soft skills and environmental awareness questionnaires. The soft skills questionnaire comprised six sub-skills questions: communication, collaboration, problem-solving, creativity, teamwork, and social interaction. The environmental awareness questionnaire contained questions about the PSTs' attitudes, behavior, and willingness to act. These sub-skills were developed based on literature studies and reviews of previous studies. Face validity was conducted by the experts, and Cronbach's alpha coefficients of 0.84 (for soft skills) and 0.86 (for environmental awareness) were obtained. The data were analyzed using the Wilcoxon signed-rank test. The result showed that SSI-based instruction had a significant effect on improving preservice science teachers' soft skills and environmental awareness. After being taught using SSI-based instruction, all the sub-skills of the soft skills of the experimental group had increased compared to before. A similar trend was also found in their scores for environmental awareness. All the components of environmental awareness, after lecturing using SSI-based instruction, showed a significant increase compared to before. These results support the claim that SSI-based instruction fosters soft skills and promotes environmental awareness.