It is unknown whether the use of intravascular ultrasound (IVUS) guidance during percutaneous coronary intervention can attenuate the stent length effect on clinical outcomes. The aim of the present study was to determine the differential prognostic effect of IVUS according to the implanted stent length. We enrolled 3,244 consecutive patients from the Interventional Cardiology Research In-cooperation Society-Drug-Eluting Stents (IRIS-DES) registry who had undergone single or overlapping stent implantation. The primary end point was major adverse cardiac events (MACE; a composite of death, myocardial infarction, and target vessel revascularization). The study population was divided by the tertiles of implanted stent length and IVUS usage. IVUS use was at the discretion of the operator. After adjusting for significant covariates, the stent length was significantly associated with the risk of MACE in the no-IVUS group (hazard ratio 1.13, 95% confidence interval 1.01 to 1.28, p = 0.042) but not in the IVUS group (hazard ratio 1.08, 95% confidence interval 0.97 to 1.20, p = 0.16). In addition, in patients with an implanted stent length of ≤22 mm (n = 998), the risk of MACE was not significantly different between the IVUS group and the no-IVUS group (hazard ratio 1.06, 95% confidence interval 0.50 to 2.28, p = 0.88). In contrast, in patients with a longer implanted stent length, the risk of MACE was significantly lower in the IVUS group than in the no-IVUS group (hazard ratio 0.47, 95% confidence interval 0.24 to 0.92, p = 0.027 for 23 to 32 mm, n = 1,109; hazard ratio 0.57, 95% confidence interval 0.33 to 0.98, p = 0.042 for ≥33 mm, n = 1,137). In conclusion, IVUS usage can attenuate the detrimental effect of the increase in the implanted stent length, supporting IVUS usage, particularly during percutaneous coronary intervention with long stent implantation.