Background: Mechanical low back pain (MLBP) is a leading cause of disability worldwide, with well-established risk factors such as obesity, occupational ergonomics, and core muscle strength. However, the role of pes planus (flat foot) as a contributing factor remains underrecognized. This study aims to investigate the association between flat foot and MLBP using Clarke’s angle as an objective measure of foot posture. Methods: A case-control study was conducted in South Timor Tengah Regency, East Nusa Tenggara, Indonesia, from December 2024 to February 2025. Fifty patients diagnosed with MLBP and 50 healthy controls were enrolled. Foot type assessment was performed using Clarke’s angle, with a cutoff of ≤30° indicating flat foot. Pain severity in the LBP group was recorded using the Numerical Rating Scale (NRS). Statistical analysis was conducted using chi-square and independent t-tests, with significance set at p < 0.05. Results: Flat foot was significantly more prevalent in the LBP group (58%) than in the control group (18%) (p = 0.000; OR: 6.29, 95% CI: 2.52-15.69), indicating that individuals with flat feet are over six times more likely to experience MLBP. No significant differences were observed between the groups regarding BMI, age, or gender. Conclusion: These findings suggest that flat foot is an independent risk factor for MLBP, likely due to altered spinal biomechanics and compensatory postural changes. Clinicians should consider foot posture assessments in MLBP patients and explore targeted interventions, such as orthotic support, to mitigate symptoms.