Purpose: Burst abdomen is a serious complication requiring immediate surgical treatment. This study aimed to investigate the association between rectus diastasis and burst abdomen in patients undergoing emergency midline laparotomy. Methods: A single-center, retrospective, matched case–control study of patients undergoing emergency midline laparotomy from May 2016 to August 2021 was conducted. Cases (patients who suffered from burst abdomen) were matched 1:4 with controls based on age and sex. Rectus diastasis was evaluated on CT imaging and was defined as a distance of at least three centimeters between the rectus abdominis muscles, three centimeters above the umbilicus. Midline laparotomy aponeurosis closure was standardized during the study period, using a slowly absorbable suture, sutured continuously with small bites of five millimeters and a minimum suture-to-wound ratio of 4:1. The primary outcome was the association between rectus diastasis and burst abdomen, evaluated against other suspected risk factors including obesity, liver cirrhosis, previous laparotomy, midline hernias and active smoking in a multivariate analysis. Results: A total of 465 patients were included in the study, with 93 cases matched to 372 controls. Eighty-four patients had rectus diastasis (35.5% cases vs. 13.7% controls; p = < 0.001). Multivariate analysis found rectus diastasis significantly associated with burst abdomen (OR 3.06, 95% CI 1.71–5.47; p = < 0.001). No other suspected risk factors showed a significant association with burst abdomen. Conclusion: Rectus diastasis was highly associated with an increased risk of burst abdomen after emergency midline laparotomy.