Background: There is wide agreement nowadays that a clinical varicocoele should be ligated to treat male factor infertility. However, the significance of testicular artery preservation in patients with severe oligozoospermia has not been addressed before. Objectives: To assess the outcome of varicocelectomy in infertile men with severe oligozoospermia and clinical varicocoeles and to compare internal spermatic artery preservation vs. artery ligation. Materials and Methods: This prospective randomized study included 302 infertile patients with severe oligozoospermia and clinical (grade II/III) varicocoeles. Patients were randomized into two groups: group A (150 patients) underwent artery-preserving varicocelectomy (APV) and group B (152 patients) underwent artery-ligating varicocelectomy (ALV). The primary outcome was to assess the changes in sperm density and motility at 3 and 6 months postoperatively in both groups. The secondary outcome was to compare the natural pregnancy rate at 1-year of follow-up. Univariate and multivariate analyses were performed to determine factors affecting pregnancy rate. Results: In both groups, there was a statistically significant improvement in sperm density and motility at 3 and 6 months postoperatively. In group A, there was a greater improvement in sperm density (p < 0.001) and motility (p < 0.001) compared to group B. At 1-year follow-up, overall 35.1% achieved a natural pregnancy. Group A achieved a significantly higher natural pregnancy rate (40% vs. 30%, p value = 0.03) compared to group B. Smaller testicular volume and ALV were the independent predictors of lower pregnancy rate ((HR = 3.2, 95% CI 1.2-8.3, p = 0.01) and (HR = 3.2, 95% CI 1.4-7.1, p = 0.003), respectively). Conclusion: In men with severe oligozoospermia and a clinical varicocoele, APV results in improved outcomes as compared to ALV with respect to semen parameters and natural pregnancy rates. Therefore, all attempts should be made to preserve internal spermatic arteries (ISA) during varicocelectomy in men with severe oligozoospermia.