Appropriate responses to an imminent threat brace us for adversities. The ability to sense and predict threatening or stressful events is essential for such adaptive behavior. In the mammalian brain, one putative stress sensor is the paraventricular nucleus of the thalamus (PVT), an area that is readily activated by both physical and psychological stressors1-3. However, the role of PVT in the establishment of adaptive behavioral responses remains unclear. Here we show in mice that PVT regulates fear processing in the lateral division of the central amygdala (CeL), a structure that orchestrates fear learning and expression4,5. Selective inactivation of CeL-projecting PVT neurons prevented fear conditioning, an effect that can be accounted for by an impairment in fear conditioning-induced synaptic potentiation onto somatostatin-expressing (SOM+) CeL neurons, which has previously been shown to store fear memory6. Consistently, we found that PVT neurons preferentially innervate SOM+ neurons in the CeL, and stimulation of PVT afferents facilitated SOM+ neuron activity and promoted intra-CeL inhibition, two processes that are critical for fear learning and expression5,6. Notably, PVT modulation of SOM+ CeL neurons was mediated by activation of the brain-derived neurotrophic factor (BDNF) receptor tropomysin-related kinase B (TrkB). As a result, selective deletion of either Bdnf in PVT or Trkb in SOM+ CeL neurons impaired fear conditioning, while infusion of BDNF into CeL enhanced fear learning and elicited unconditioned fear responses. Our results demonstrate that the PVT–CeL pathway constitutes a novel circuit essential for both the establishment of fear memory and the expression of fear responses, and uncover mechanisms linking stress detection in PVT with the emergence of adaptive behavior.