Background <p>Flexor pollicis longus (FPL) tenosynovitis is an uncommon condition in adolescents and is typically associated with rheumatologic, traumatic, or infectious causes. Overuse-related FPL tenosynovitis due to faulty writing technique is not well documented in the literature.</p> Case Presentation <p>We present a rare case of a 14-year-old student with a 4-year history of FPL tenosynovitis caused by excessive thumb flexion during writing, along with a low grip position on a narrow-diameter pen. He was previously misdiagnosed with carpal tunnel syndrome and cervical radiculopathy by multiple physicians. However, careful clinical and sonographic examination revealed the correct diagnosis of FPL tenosynovitis, ruling out other differentials. Initial management with oral medication, cold packs, ergonomic corrections, and thumb spica splinting led to partial improvement. The patient later showed marked improvement following an ultrasound-guided injection of autologous platelet-rich plasma (PRP) around the FPL tendon.</p> Conclusion <p>This case highlights the importance of thorough history taking, clinical evaluation, and targeted sonographic assessment in recognizing writing-induced FPL tenosynovitis in adolescents. Early diagnosis and appropriate intervention are essential to prevent long-term disability. Additionally, PRP may serve as a safe and effective adjunct in chronic, non-responsive cases, particularly in skeletally immature individuals where corticosteroid use is controversial.</p>