The oxidative coupling of methane (OCM) is a promising pathway for directly converting methane into higher hydrocarbons (C₂₊). This research investigated the influence of alkali metal promoters (Li, Na, K, or Rb) on Co/Al₂O₃ catalysts prepared based on incipient wetness impregnation for the OCM reaction. The catalyst investigations demonstrated that the catalysts promoted with K and Rb had superior performance, with the 4.6K-Co/Al₂O₃ catalyst achieving a maximum C₂₊ yield of 8.1%, C₂₊ selectivity of 24.0%, and CH₄ conversion of 32.1% at 640 °C. Catalyst characterization, based on XRD, HR-TEM, BET, XPS, CO₂-TPD, and H₂-TPR analyses, revealed the structural and physicochemical properties responsible for the enhanced catalytic activity. Specifically, K and Rb promoters increased surface basicity and enhanced the electron density of active sites, thereby promoting selective methane activation. <i>In-situ</i> DRIFTS and mechanistic studies highlighted the role of reactive oxygen species in promoting C₂₊ hydrocarbon formation. These results should position K-Co/Al₂O₃ as a promising catalyst for OCM and provide valuable guidance for designing more efficient catalytic systems for methane utilization.