Objectives To examine the effects of single- and double-layer uterine closure techniques on uterine scar healing following cesarean delivery. Design Randomised controlled trial Setting University Hospital of Başkent from July 2018 to September 2019 Population This study assessed a total of 282 women between 18-45 years of age who had singleton pregnancies and had not previously undergone uterine surgeries. Methods Participants were randomized into two following treatment groups: single-layer closure with locking and double-layer closure with locking in the first layer, but not in the second layer (NCT03629028). Participants were evaluated at 6-9 months after cesarean section by saline infusion sonohysterography to assess cesarean delivery scar defects. Experienced sonographers who were not informed about the uterine closure technique conducted these procedures. Results Of the 225 final participants, 116 received the double-layer closure technique, while 109 received the single-layer technique. In this regard, the niche rates and median niche depths based on transvaginal ultrasounds and sonohysterography investigations were 21%, 0.9±1.8mm, and 41%, 2.1±1.9mm (p<0.001, p<0.001), respectively. The niche rates were 37% for the single-layer group and 45.7% for the double-layer group (p=0.22). Median niche widths were higher in the double-layer group (p=0.006). Conclusions The single- and double-layer closure techniques did not produce different impacts on uterine scare niche development. However, the median niche width was higher for the double-layer closure group. Tweetable abstract There was no difference between single- and double layer closure in the rate of scar niche development following cesarean delivery. Keywords cesarean delivery, isthmocele, niche, uterine closure