Background and Purpose Ischemic core estimation by CT perfusion (CTp) is a diagnostic challenge, mainly because of the intrinsic noise associated with perfusion data. However, an accurate and reliable quantification of the ischemic core is critical in the selection of patients for reperfusion therapies. Our study aimed at assessing the diagnostic accuracy of two different CTp postprocessing algorithms, that is, the Bayesian Method and the oscillation index singular value decomposition (oSVD). Methods All the consecutive stroke patients studied in the extended time window (>4.5 hours from stroke onset) by CTp and diffusion‐weighted imaging (DWI), between October 2019 and December 2021, were enrolled. The agreement between both algorithms and DWI was assessed by the Bland‐Altman plot, Wilcoxon signed‐rank test, Spearman's rank correlation coefficient, and the intraclass correlation coefficient (ICC). Results Twenty‐four patients were enrolled (average age: 72 ± 15 years). The average National Institutes of Health Stroke Scale was 14.42 ± 6.75, the median Alberta Stroke Program Early CT score was 8.50 (interquartile range [IQR] = 7.75‐9), and median time from stroke onset to neuroimaging was 7.5 hours (IQR = 6.5‐8). There was an excellent correlation between DWI and oSVD (ρ = .87, p‐value < .001) and DWI and Bayesian algorithm (ρ = .94, p‐value < .001). There was a stronger ICC between DWI and Bayesian algorithm (.97, 95% confidence interval [CI]: .92‐.99, p‐value < .001) than between DWI and oSVD (.59, 95% CI: .26‐.8, p‐value < .001). Discussion The agreement between Bayesian algorithm and DWI was greater than between oSVD and DWI in the extended window. The more accurate estimation of the ischemic core offered by the Bayesian algorithm may well play a critical role in the accurate selection of patients for reperfusion therapies.