Background/Objectives: Tenosynovial giant cell tumor (TGCT), historically referred to as pigmented villonodular synovitis (PVNS), is a rare proliferative synovial disorder that most commonly affects the knee and may lead to progressive joint damage. Surgical synovectomy remains the mainstay of treatment, although recurrence remains a concern, particularly in diffuse disease. The purpose of this study was to evaluate functional outcomes and recurrence following arthroscopic synovectomy with selective open excision for TGCT of the knee. Methods: This retrospective case series included patients treated surgically for TGCT of the knee at a single tertiary orthopaedic center between 2003 and 2023. The extended study period may have introduced variability in surgical technique and perioperative management due to evolving clinical practice. All patients underwent arthroscopic synovectomy, with open excision performed when necessary to address posterior or extra-articular disease not amenable to complete arthroscopic resection. Functional outcomes and knee range of motion were evaluated preoperatively and at final follow-up. Recurrence and complications were recorded. Results: A total of 43 patients were included with a mean follow-up of 8.1 years. Diffuse TGCT was present in 32 patients (74%), while 11 patients (26%) had localized disease. Arthroscopic synovectomy was performed in all cases, with additional open excision required in 10 patients. Functional scores and range of motion improved significantly at final follow-up (p < 0.001). Disease recurrence occurred in 8 patients (18.6%), all with diffuse disease, and was treated with revision arthroscopic synovectomy. Conclusions: Arthroscopic synovectomy with selective open excision when required was associated with significant improvement in functional outcomes and knee range of motion. Disease recurrence occurred in 18.6% of patients and exclusively in those with diffuse disease, emphasizing the need for careful postoperative surveillance.