Flooding associated with rising inland lake levels represents a distinct and understudied hazard compared to conventional river or coastal flooding. Unlike riverine floods, lake inundation develops gradually and persists over extended periods, yet integrated flood risk assessments specifically addressing this phenomenon remain scarce. This study uses Lake Baringo in Kenya’s Rift Valley as a case study, a lake that has experienced exceptional water level rises of nearly 10 m since 2010, to assess flood risk through the integration of scenario-based flood hazard modelling, spatial exposure assessment, and household vulnerability analysis. Three lake level scenarios were developed, spanning from maximum observed conditions to a worst-case threshold at the lake’s sill point. Results show that under the worst-case scenario, the lake area could expand by approximately 64%, potentially exposing nearly 18,000 people and 156 km of road infrastructure. A Flood Vulnerability Index (FVI) ranging from 0.58 to 0.73 indicated consistently high socioeconomic vulnerability across all shoreline communities, with vulnerability accounting for 42–53% of overall flood risk across scenarios. Flood risk was highest along the flat southern shoreline, where topography amplifies lateral inundation extent. The findings highlight the importance of integrating lake level dynamics into spatial planning and flood risk management, and the need for improved monitoring, livelihood diversification, and institutional coordination in semi-arid inland lake regions facing increasing hydro-climatic variability.