This study sought to investigate the predictors and outcomes of side branch (SB) occlusion after main vessel (MV) stenting in coronary bifurcation lesions. SB occlusion is a serious complication that occurs during percutaneous coronary intervention (PCI) for bifurcation lesions. Consecutive patients undergoing PCI using drug-eluting stents for bifurcation lesions with SB ≥2.3 mm were enrolled. We selected patients treated with the 1-stent technique or MV stenting first strategy. SB occlusion after MV stenting was defined as Thrombolysis in Myocardial Infarction flow grade <3. SB occlusion occurred in 187 (8.4%) of 2,227 bifurcation lesions. In multivariate analysis, independent predictors of SB occlusion were pre-procedural percent diameter stenosis of the SB ≥50% (odds ratio OR: 2.34; 95% confidence interval CI: 1.59 to 3.43; p < 0.001) and the proximal MV ≥50% (OR: 2.34; 95% CI: 1.57 to 3.50; p < 0.001), SB lesion length (OR: 1.03; 95% CI: 1.003 to 1.06; p = 0.03), and acute coronary syndrome (OR: 1.53; 95% CI: 1.06 to 2.19; p = 0.02). Of 187 occluded SBs, flow was restored spontaneously in 26 (13.9%) and by SB intervention in 103 (55.1%) but not in 58 (31.0%). Jailed wire in the SB was associated with flow recovery (74.8% vs. 57.8%, p = 0.02). Cardiac death or myocardial infarction occurred more frequently in patients with SB occlusion than in those without SB occlusion (adjusted hazard ratio: 2.34; 95% CI: 1.15 to 4.77; p = 0.02). Angiographic findings of SB, proximal MV stenosis, and clinical presentation are predictive of SB occlusion after MV stenting. Occlusion of sizable SB is associated with adverse clinical outcomes..