Adapting to changing environmental conditions requires a prospective inference of future actions and their consequences, a strategy also known as model-based decision making. In stable environments, extensive experience of actions and their consequences lead to a shift from a model-based to a model-free strategy, whereby behavioral selection is primarily governed by retrospective experiences of positive and negative outcomes. Human and animal studies, where subjects are required to speculate about implicit information and adjust behavioral responses over multiple sessions, point to the central serotonergic system's role in model-based decision making. However, to directly test a causal relationship between serotonergic activity and model-based decision making, phase-specific manipulations of serotonergic activity are needed in a one-shot test, where learning by trial and error is neutralized. Moreover, the serotonergic origin responsible for this effect is yet to be determined. Herein, we demonstrate that optogenetic silencing of serotonin neurons in the dorsal raphe nucleus, but not in the median raphe nucleus, disrupts model-based decision making in lithium-induced outcome devaluation tasks. Our findings provide insights into the neural mechanisms underlying neural weighting between model-free and model-based strategies.