IIntentional emotional contagion, the conscious transmission of emotions between individuals, has unclear neurobehavioral mechanisms in naturalistic dyadic communications. Experiment 1 demonstrated that conscious engagement enhanced sad emotional contagion between strangers. Using the fNIRS pseudo-hyperscanning technique, Experiment 2 showed a behavioral-neural dissociation in intentional sadness contagion: Behavioral findings revealed increased interpersonal intimacy and listener altruism. Neurocognitive findings demonstrated reduced activation in the left frontopolar cortex, bilateral dorsolateral prefrontal cortex (dlPFC), and angular/supramarginal gyri. While weaker interpersonal neural synchronization emerged between the speaker’s left frontopolar/dlPFC and listener’s right dlPFC. Mediation analysis indicated that intentional emotional contagion mediated emotional communication and self-other overlap, extending Emotions as Social Information Model. Findings suggest that intentional sadness contagion based on dyadic story sharing has an adaptive neurocognitive reorganization which facilitates prosocial behavior.