  Two-dimensional topological insulators (2DTIs), which host the quantum spin
Hall (QSH) effect, are one of the key materials in next-generation spintronic
devices. To date, experimental evidence of the QSH effect has only been
observed in a few materials, and thus, the search for new 2DTIs is at the
forefront of physical and materials science. Here, we report experimental
evidence of a 2DTI in the van der Waals material Ta$_2$Pd$_3$Te$_5$.
First-principles calculations show that each monolayer of Ta$_2$Pd$_3$Te$_5$ is
a 2DTI with weak interlayer interactions. Combined transport, angle-resolved
photoemission spectroscopy, and scanning tunneling microscopy measurements
confirm the existence of a band gap at the Fermi level and topological edge
states inside the gap. These results demonstrate that Ta$_2$Pd$_3$Te$_5$ is a
promising material for fabricating spintronic devices based on the QSH effect.
