To date, no studies have evaluated the outcomes of lumbar interbody fusion using the anterior to psoas (ATP) approach in patients with spondylolisthesis at L5/S1. We; therefore, aimed to evaluate short-term surgical outcomes of interbody fusion using the ATP approach combined with posterior fixation in these patients.We performed a retrospective analysis of 9 patients with grade I spondylolisthesis at L5/S1 who were treated with fusion and posterior fixation using the ATP approach at our hospital from April to July 2018. The recorded parameters included operation time, intraoperative blood loss, complications, intervertebral fusion rate, radiological intervertebral height, intervertebral foramen height, intervertebral foramen width, pain, visual analog scale, and Oswestry disability index.Four men and 5 women at an average age of 57.8 years (range: 46-71 years) were enrolled in the study. The average operation time was 152.8 ± 22.9 minutes, and the average blood loss during surgery was 165 ± 27.5 mL. All patients confirmed the relief of their low back pain, and there were no serious complications. The follow-up time was more than 6 months. The visual analog scale and Oswestry disability index scores 3 days postoperatively and at the last follow-up were significantly lower than those before surgery (P < .05). At the last follow-up, the intervertebral space of the surgical segment showed bony fusion in all patients, and the intervertebral height and intervertebral foramen height and width were significantly increased compared with those before surgery (P < .05).The ATP approach was safe and effective for the treatment of spondylolisthesis at L5/S1. It showed low vascular injury and cage shift rates and was technically easy to perform. We recommended that surgeons identify the vessels in the surgical field preoperatively so that they can be secured or safely ligated during surgery.