Purpose: The presented study was conducted to unpack high school students' motivational influences in engineering/computer science project-based learning (PjBL), using the attention, relevance, confidence, and satisfaction (ARCS) model of motivation as a conceptual framework. Methods: A qualitative research approach was used with student focus groups as the data source. A total of six focus groups with 32 student participants was conducted. The students were enrolled in high schools located in four different states in the U.S. The qualitative analysis of transcripts was performed using first and second cycle coding methods. Findings: The findings show that student motivation is nuanced in regard with attention, relevance, confidence, and satisfaction. The findings identify research-based strategies for fostering student motivation, such as implementing learner-focused scaffolding in PjBL environments, improving the relevance of the classroom content with the real-world context that students have experiences in or are knowledgeable about, and focusing on stimulating intrinsic motivation in addition to extrinsic rewards. Conclusion: The findings provide support for the comprehensibility and utility of the ARCS model of motivation in high school engineering/CS education, and more importantly empirically unpacks what the model factors mean from students' perspective. Practitioners may use these findings to inform the design, development, and implementation of PjBL in high school settings.