Background: One of the methods of stabilizing the stump of the ulna following resection of the distal ulna is tenodesis of the extensor carpi ulnaris (ECU). Some studies have recommended stabilization, whereas others have not found it useful. Most of these studies have a mix of different pathologies and often do not have a control group. The aim of this study is to compare the outcomes of ECU tenodesis versus no tenodesis after resection of the distal ulna in patients with grade III giant cell tumor (GCT) of the distal ulna. Methods: The retrospective study included 10 patients with Campanacci grade III GCT of the distal ulna treated by resection of the distal ulna between 2014 and 2019. Patients were stratified into two groups based on whether they underwent ECU tenodesis (n = 5) or no tenodesis (n = 5). The patients were assessed at 6 weeks, 6 months, and 12 months for complications and outcomes using the Mayo wrist score (MWS) and the revised musculoskeletal tumor society score (MSTS). Results: The MWS and the MSTS were significantly better in the ECU tenodesis group at 6 weeks. At 6 months, MWS was similar in both groups, but MSTS continued to be significantly better in ECU tenodesis group. At 12 months, both groups reported similar MWS and MSTS. There were no recurrences in either groups. One patient in the ECU tenodesis group developed ECU tendonitis that resolved with conservative treatment. Conclusions: The outcomes of ECU tenodesis were better in the short term (6 months), although both groups reported similar outcomes at 12 months. Level of Evidence: Level III (Therapeutic).