  Two-dimensional (2D) materials, with their structural uniqueness, exceptional
properties, and wide-ranging applications, show unprecedented prospects in
fundamental physics research and industrial applications. 2D \b{eta}-phase
cuprous iodide (\b{eta}-CuI) is a promising candidate for overcoming the
challenges of insufficient P-type transparent conductive materials, with
multiple predicted unique properties. Previous experimental results show that
\b{eta}-CuI only occurs at elevated temperatures between 645 and 675 K. Many
efforts are made to stabilize \b{eta}-CuI at room temperature through
surface/interface engineering. However, the resulting composites limit the
performance and application of pure 2D \b{eta}-CuI. Here, we demonstrate
experimentally that isolated 2D \b{eta}-CuI single crystals can exist stably
under ambient conditions, a high-temperature phase CuI found at room
temperature. We validate the simultaneous existence of {\gamma}-CuI and
\b{eta}-CuI in the synthesized CuI. The previous neglect of \b{eta}-CuI
crystals can be ascribed to factors including their low content, small
dimensions, and lack of ingenious experimental characterization. Moreover, the
theoretical calculation further confirms dynamically and thermally stable of
the monolayer \b{eta}-CuI, which has an ultra-wide direct band-gap (3.66 eV).
Our findings challenge the traditional understanding of \b{eta}-CuI as a
high-temperature phase of CuI, instead providing a new definition that 2D
\b{eta}-CuI exhibits remarkable stability under ambient conditions.
