The effectiveness of science problem-based learning (PBL) is highly dependent on individual students' variability. Researchers have shown the need for cognition (NFC) and motivation are significant factors. Despite the acknowledged impact of these constructs, there is a gap in understanding the relationships among NFC, motivation, and learning outcomes in science PBL for diverse students. To fill this gap, we conducted a correlational study in the southwestern United States and examined the influence of NFC on middle school students' (n = 478) motivation and learning outcomes in science PBL, taking into account their varying NFC levels (measured by Cacioppo et al., 1984), motivation (measured by Ryan & Deci, 2000), race, and gender. Confirmatory factor analysis, two-step structural equation modeling (SEM), and multigroup SEM were used to analyze the data. The results indicated that NFC played a critical role in boosting motivation for both science learning in general and science PBL, ultimately leading to improved science knowledge. However, the direct link between motivation specific to science PBL and knowledge gains was statistically nonsignificant, suggesting other unmeasured factors may also influence how science PBL influences learning outcomes. Additionally, the significant impact of NFC on motivation for science learning was detected in the high NFC group but not in the low NFC group. Regarding the possible moderation of race and gender factors, only race showed a partial moderation effect. This study contributes to a more nuanced understanding of how NFC, motivation, and socio-demographic factors influence learning outcomes, offering directions that may foster more inclusive and effective science education strategies.