Background: Peritoneal lavage is a need after laparatomies performed for secondary peritonitis. Some previous studies mentioned the benefit of abdominal lavage with antibiotics, in secondary peritonitis operations. The current study aimed to compare normal saline alone (NS) with normal saline plus gentamicin (NS + G) for abdominal lavage in secondary peritonitis. Methods: In this randomized clinical trial (RCT), patients who were referred to the emergency department, and were candidates for urgent laparotomy were enrolled. After giving informed consent, 80 patients were randomized into 2 groups of peritoneal lavage with (NS) and (NS + G). Descriptive statistics were calculated for the presented data. Chi-square test and Fisher’s exact test was used for determining the association between qualitative variables. Comparison between the groups was made using the nonparametric Mann-Whitney test. The Kolmogorov–Smirnov test was used for normality of the data. Results: Themean age of the patients was 41.39 years, and 67.5% weremale. There were no significant differences in the demographic data, history of abdominal surgery, diabetes, and the cause of peritonitis. Twenty-one patients needed another surgical intervention. The need for surgical interventions was significantly more in the NS group compared to the NS + G group (17.5% vs. 35%, P = 0.039). There was no significant difference in fever, wound infection, admission time, intravenous antibiotic duration, return to work time, and mortality between the 2 groups. Conclusions: It seems that peritoneal lavage with gentamicin in secondary peritonitis may decrease the need for surgical interventions.