Agriculture in sub-Saharan Africa (SSA) countries is predominantly rainfed, but SSA could develop greater dependence on supplemental irrigation due to a changing climate with greater rainfall uncertainty and higher frequency of dry spells. Supplemental irrigation through small-scale water harvesting (SSWH) plays a vital role in helping rainfed small-scale farmers overcome the risk of dry spells and promotes greater investment in agriculture. This study employs a contingent valuation approach to estimate the demand for SSWH supplemental irrigation in Ghana. The study finds the mean willingness-to-pay estimates for SSWH supplemental irrigation to be GHC 25.36 (USD 6.67) per acre per season for open canal irrigation system and 24.76 (USD 6.52) per acre per season for pipeline irrigation system. Drought experience, access to credit, agricultural income, and land ownership are key determinants of the demand for SSWH supplemental irrigation. The findings are particularly important for pricing small-scale irrigation services from rainwater harvesting public water reservoirs. The study recommends that investing in rainwater harvesting in rainfed agriculture should form a cornerstone of any country's strategy for adapting to drought, particularly in developing countries where rainfed agriculture plays an important economic role.