Purpose <p>The role of deltoid ligament (DL) repair in supination-external rotation (SER)-type trimalleolar equivalent ankle fractures (TEAFs) remains controversial. Additionally, it remains unclear whether posterior malleolar fixation alone can prevent postoperative syndesmotic widening. Therefore, this study aimed to determine the optimal combination of surgical treatments for SER-type TEAFs involving the lateral and posterior malleoli and the DL.</p> Methods <p>We retrospectively evaluated 77 patients who underwent surgical treatment for SER-type TEAFs involving Weber type B lateral malleolar fractures with associated posterior malleolar fractures and DL injuries between 2010 and 2023. The patients were divided into DL repair (n = 32) and no repair (n = 45) groups. All the patients underwent open reduction and internal fixation of the lateral and posterior malleoli without syndesmotic fixation. Radiographic parameters, including the medial clear space (MCS), talar tilt (TT), tibiofibular clear space (TFCS), and tibiofibular overlap (TFO), were assessed on standing, gravity stress, and valgus stress radiographs. Clinical outcomes were evaluated using the Foot and Ankle Ability Measure (FAAM).</p> Results <p>At a minimum follow-up of 2 years, no significant differences were observed between the groups in any radiographic parameters on standing, gravity stress, or valgus stress views (<i>P</i> &gt; 0.05). When compared with the contralateral side, both groups demonstrated significantly increased TFCS and decreased TFO on standing mortise views (<i>P </i>&lt; 0.05), whereas MCS and TT remained comparable. The FAAM-Activities of Daily Living and FAAM-Sports scores improved significantly in both groups with no intergroup differences at the final follow-up. The DL repair group had a longer operative time and experienced DL repair-related local irritation in 11% of cases.</p> Conclusions <p>In SER-type TEAFs, routine DL repair does not confer additional radiographic or clinical benefits when stable fixation of the lateral and posterior malleoli is achieved. Residual syndesmotic widening after posterior malleolar fixation did not result in medial instability, valgus TT, or post-traumatic valgus ankle arthritis. These findings suggest that accurate bone reduction alone may be sufficient for this injury pattern without routine DL repair.</p> Level of evidence III <p>Retrospective comparative study.</p>