family*

Nontrivial states in topological semimetals are attracting intense interest driven by their numerous novel properties such as high bulk carrier mobility, Fermi arcs, chiral anomaly, large negative magnetoresistance, and high density of states. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES) along with first-principles calculations, we report the discovery of a topological nodal-loop (TNL) state in CaAs₃ family. Our analysis reveals that surface projections of the bulk nodal-points are connected by surface states. Notably, the observed topological state in CaAs₃ family are well separated from other bands in the vicinity of the Fermi level. CaAs₃ family thus offers a unique materials realization of a prototype nodal-loop semimetal, and a platform for gaining a deeper understanding of the interplay between TNL and topological insulator physics in semimetals.