${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ has attracted much recent attention as the first quasi-two-dimensional (2D) kagome superconductor. While the kagome layers are 2D in nature, increasing evidence has pointed to the importance of out-of-plane correlation in this material. However, it remains unclear whether such correlation can change the fundamental electronic structure of the quasi-2D system. Here, we reveal this missing piece of information, using angle-resolved photoemission spectroscopy, complemented by scanning tunneling microscope measurements. The three-dimensional electronic structures in the high-temperature state are revealed, which agree well with density-functional theory calculations. Electron energy bands are observed in the low-temperature state that exhibit additional periodicities along the out-of-plane momentum. These results reveal a direct response to the out-of-plane electronic supermodulation in the single-particle spectral function of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, thus establishing an electronic platform to examine emergent phenomena beyond 2D limit in kagome superconductors.

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• Topological phases of matter
• Kagome lattice
• Single crystal materials
• Angle-resolved photoemission spectroscopy
• First-principles calculations