Understanding factors that influence k-12 students' Science, Technology, Engineering, and Mathematics (STEM) performance is essential to improving their problem-solving ability. Most studies have focused on the relationship between students' psychological factors and STEM performance and have paid little attention to the relationship between behavioral factors and STEM performance. This study explored the impact of behavioral factors (i.e., information and communications technology (ICT) readiness and online interaction (OI)) and psychological factors (i.e., internet self-efficacy (ISE)) on k-12 students' STEM performance. The sample included 851 fifth graders and 535 eighth graders from cities in central China. The results of structural equation modeling analysis showed that ISE and ICT readiness (IR) significantly impacted the STEM performance of eighth graders. More importantly, ISE, a psychological factor, had the greatest effect on STEM performance and played a mediating role in the relationship between IR, OI, and STEM performance. These findings have important implications for STEM teachers. To improve students' STEM performance, teachers should intervene to improve ISE according to students' grades and cognitive ability, guide students to use ICT correctly, and encourage them to actively engage in OI.