Palmoplantar hyperkeratosis and early-onset aggressive periodontitis are the hallmarks of Papillon–Lefèvre syndrome (PLS), a rare autosomal recessive condition caused by pathogenic variants in the CTSC gene. We report a 9-year-old girl who was born to consanguineous parents and presented with progressive periodontal disease, nail dystrophy, and transgradient palmoplantar keratoderma. Based on clinical and histopathologic findings, she was initially misdiagnosed with psoriasis. Topical and systemic treatments were administered, with only mild improvement observed. A homozygous CTSC variant (c.899G>A; p.Gly300Asp) was subsequently found by whole-exome sequencing, confirming PLS. Partial clinical improvement was achieved with supportive dermatological care and multidisciplinary dental treatment. Early genetic diagnosis allows for appropriate management, and prevents unnecessary systemic immunosuppression. This case highlights the importance of considering PLS in children presenting with early-onset keratoderma and periodontal disease.