This paper examines the causal impact of e-money adoption on cash demand in Indonesia, an emerging economy where digital payments are expanding rapidly but cash remains widely used. Using a high-frequency provincial panel dataset, we address endogeneity in e-money adoption by employing a Bartik-style shift-share instrumental variable approach. We find clear evidence of substitution: a 1% increase in e-money transactions is associated with a 1.7% decline in cash demand. This effect is substantially stronger in more urbanized and digitally advanced provinces in the Java region, indicating persistent regional disparities. Evidence from cash denominations shows that substitution is concentrated among small-to medium-value banknotes typically used for everyday transactions, while coins remain largely unaffected. This study has important implications for monetary policy, particularly for managing the money supply and setting policy rates in an increasingly cashless economy, as well as for promoting targeted digital inclusion initiatives. Copyright © 2026. Published by Elsevier B.V.