Mechanical stress applied to the lower extremities during walking is a major contributor to the development and progression of knee osteoarthritis (KOA). Adequate shock absorption during gait is essential for reducing joint loading; however, excessive midfoot pronation, characterized by deformation of the medial longitudinal arch, may impair this function. Despite its biomechanical importance, quantitative assessment of midfoot pronation during walking remains challenging because conventional motion capture systems are costly, complex, and impractical for large-scale or daily-life measurements. The purpose of this study was to develop and preliminarily validate a simple, wearable system capable of efficiently quantifying midfoot pronation during walking. The proposed system integrates five thin plantar pressure sensors with an airbag-based barometric sensor positioned over the navicular tuberosity. Instead of directly measuring joint kinematics, the system quantifies pressure-based medial midfoot deformation as a relative index of midfoot pronation. System reliability was evaluated by comparing its output with navicular displacement obtained using a three-dimensional motion capture system during repeated standing-sitting tasks, focusing on similarity in temporal patterns and relative displacement trends. In addition, dynamic midfoot pronation measurements during gait were compared with static foot structural characteristics―including navicular height and rearfoot alignment―obtained using a smartphone-based three-dimensional foot scanner. The developed system demonstrated temporal patterns of navicular displacement comparable to those recorded by motion capture, with correlation coefficients ranging from 0.55 to 0.64. During walking, midfoot pronation was captured from initial contact through terminal stance, with peak values typically observed between the loading response and midstance phases. Strong associations were identified between peak midfoot pronation output and static foot structure measures, including navicular height (r = 0.88, p = 0.05) and rearfoot alignment angle (r = −0.91, p = 0.03), consistent with established biomechanical relationships. These findings indicate that the proposed wearable system can effectively capture relative midfoot motion associated with pronation during walking. Although this pilot study had a small sample size, the results support the feasibility of the system as a practical tool for field-based gait analysis, with potential applications in screening, longitudinal monitoring, and large-scale studies of midfoot biomechanics and lower-limb joint loading.