Abstract The Arctic summer marginal ice zone (MIZ) represents a growing fraction of total sea ice extent (SIE), yet its predictive performance for September Arctic sea ice remains unknown. This study reveals that summer (June–August) MIZ area fraction is an optimal predictor for September Arctic SIE, particularly after 2000, with better forecast skill (S = 0.66) than summer Arctic SIE (S = 0.48). Gridded binary MIZ fields alone accurately predict September sea ice coverage (accuracy: 0.80–0.93), but overestimate the extent near the ice edge. Integrating gridded binary MIZ metrics, ice thickness, northward ice velocity, sea surface temperature, and net surface heat flux in peripheral seas mitigates such positive biases, with precision increasing by 0.09. This work highlights summer MIZ properties as key predictors for September sea ice, with predictive performance substantially improved by incorporating additional ice and oceanic variables.