In this study, sourdough powder was used as a natural additive to enhance functional properties and reduce glycemic index of corn flakes. Lactobacillus fermentum, Lactococcus lactis, previously isolated from sourdough samples, and Saccharomyces cerevisiae were used as starter cultures to produce sourdough powder from wheat flour. To produce corn flakes sourdough powder was replaced by maize flour in amounts of 15 and 30%, while the control sample contained no sourdough powder. The total phenolic content, 2,2-di-phenyl-2-picryl-hydrazyl (DPPH) radical scavenging activity and ferric reducing antioxidant power (FRAP) of the samples were 421.58 mg GAE/kg, 333.90 µM TE/g, 62.53 mg/kg for control sample, 482.41 mg GAE/kg, 350.60 µM TE/g, 82.22 mg/kg for corn flakes with 15% sourdough, and 531.10 mg GAE/kg, 368.14 µM TE/g, 117.42 mg/kg for corn flakes with 30% sourdough, respectively. Total dietary fibre content, the starch hydrolysis rate, and rapidly digestible starch (RDS) values of corn flakes samples decreased with the addition of 30% sourdough powder (P < 0.05). The estimated glycemic index (eGI), which was 95.76 for the control sample, decreased to 83.41 for the sample with 30% sourdough. The addition of sourdough had no negative effect on the sensorial properties (P > 0.05).