There are few studies regarding functional outcome and lesion extent on diffusion-weighted MRI (DWI) in patients with posterior circulation (PC) infarction. The aim of our study was to assess whether a newly proposed posterior circulation Alberta Stroke Program Early CT Score (pc-ASPECTS) on DWI is useful for predicting functional outcome in PC patients. One hundred thirty-two patients with first-ever ischemic stroke in the posterior circulation within 24 h of onset who were admitted to our hospital were enrolled in the study. We compared background characteristics, vital signs, laboratory data, and MRI findings between favorable (F) and unfavorable (U) outcome groups at 3 months, according to the modified Rankin Scale (mRS). The F and U groups were defined as having a mRS of 0-2 and 3-6, respectively. pc-ASPECTS was scored by DWI obtained 12-36 h after onset. Ninety-eight patients (74.2%) were classified into the F group and 34 patients (25.8%) into the U group. On univariate analysis, F group patients were younger, had lower National Institutes of Health Stroke Scale (NIHSS) score at entry, and a lower rate of early neurological deterioration (END) and cardioembolic stroke than U group patients. On MRI, F group patients had lower leukoaraiosis and medial temporal atrophy score and higher pc-ASPECTS score on DWI compared to U group patients. Multiple logistic regression analysis revealed NIHSS (p < 0.001), END (p = 0.0057), pc-ASPECTS (p < 0.001), and leukoaraiosis (p = 0.0091) as independent predictors of functional outcome. pc-ASPECTS appears to be a powerful marker for predicting functional outcome, along with clinical severity and END. Leukoaraiosis may also be an independent predictor of functional outcome.