BACKGROUND: The relevance of closing the platysma muscle layer after open thyroidectomy has received little interest in terms of research. The objective was to determine whether non-closure of the platysma muscle layer after open thyroidectomy impacts significantly on the cosmetic outcome of the resulting collar scar. METHODS: In this prospective randomised controlled clinical trial, patients were assigned randomly to have the platysma muscle layer closed or not closed. The primary endpoint was the cosmesis of the collar scar six weeks after surgery assessed using the patient and observer scar assessment scale (POSAS). Additional endpoints included operation time and early postoperative wound complications. RESULTS: Ninety-two patients were recruited, with 46 randomised to each group. The patient scar assessment subscale (PSAS) of the POSAS showed no significant difference in the scar cosmesis between the two groups six weeks after surgery (median PSAS: 16.5 vs 17.5; p = 0.514). The observer scar assessment subscale (OSAS) showed that the platysma muscle layer closure group had marginally better scars (median OSAS: 15 vs 17; p = 0.045). The size of the goitre did not make any significant difference in the scar cosmesis. There was no significant difference in the incidence of early postoperative wound complications as well as the median operation time. CONCLUSION: Not closing the platysma muscle layer had no significant impact on the scar cosmesis six weeks after open thyroidectomy, with no significant difference in the incidence of early postoperative wound complications and the operation time.