Interactions between the thalamus and cerebral cortex are crucial for relaying sensory signals, and their impediment is associated with neuropsychiatric disorders. However, the pathogenesis of these disorders, including autism spectrum disorder, remains unsolved due to the lack of in vitro models that mimic pathophysiological events of the human brain. Brain organoids, three-dimensional cell aggregates differentiated from pluripotent stem cells, have been shown to partly mimic the structure, function, and development of some brain regions in vitro. Here we report in vitro thalamocortical and corticothalamic interactions by generating assembloids, a 3D assembly of organoids, from human induced pluripotent stem cells (hiPSCs). We differentiated hiPSCs to both thalamic organoids and cortical organoids, each of which expressed brain region-specific markers. We then generated assembloids by fusing the thalamic and cortical organoids. Labeling the organoids with fluorescent proteins visualized reciprocal projections in the assembloids. In addition, rabies viral tracing demonstrated transsynaptic labeling between two organoids, suggesting the formation of synaptic connections in the assembloids. The in vitro models of neural circuits between the thalamus and cortex will help us understand neuropsychiatric disorders.