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The excitonic insulator (EI) is an exotic ground state of narrow-gapsemiconductors and semimetals, arising from spontaneous condensation ofelectron-hole pairs bound by attractive Coulomb interaction. Despite researchon EIs dating back to half a century ago, their existence in real materialsremains a subject of ongoing debate. In this study, through systematicexperimental and theoretical investigations, we have unveiled the existence ofan EI ground state in the van der Waals compound Ta2Pd3Te5. Density functionaltheory calculations suggest that it is a semimetal with a small band overlap,whereas various experiments exhibit an insulating ground state with a clearband gap. Upon incorporating electron-hole Coulomb interaction into ourcalculations, we obtain an EI phase where the electronic symmetry breakingopens a many-body gap. Angle-resolved photoemission spectroscopy measurementsdemonstrate that the band gap is closed with a significant change in thedispersions when charge carriers sufficiently screen the electron-hole Coulombinteraction. Structural measurements reveal a slight breaking of crystalsymmetry with exceptionally small lattice distortion in the insulating state,resulting from the coupling to the symmetry-breaking electronic state. Ourfindings provide compelling evidence that the insulating ground state with agap opening in Ta2Pd3Te5 originates from exciton condensation driven byelectron-hole coupling.

Evidence for an Excitonic Insulator State in Ta2Pd3Te5