  Thinning crystalline materials to two dimensions (2D) creates a rich
playground for electronic phases, including charge, spin, superconducting, and
topological order. Bulk materials hosting charge density waves (CDWs), when
reduced to ultrathin films, have shown CDW enhancement and tunability. However,
charge order confined to only 2D remains elusive. Here we report a distinct
charge ordered state emerging in the monolayer limit of $1T$-VSe$_2$.
Systematic scanning tunneling microscopy experiments reveal that bilayer
VSe$_2$ largely retains the bulk electronic structure, hosting a
tri-directional CDW. However, monolayer VSe$_2$ -- consistently across distinct
substrates -- exhibits a dimensional crossover, hosting two CDWs with distinct
wavelengths and transition temperatures. Electronic structure calculations
reveal that while one CDW is bulk-like and arises from the well-known Peierls
mechanism, the other is decidedly unconventional. The observed CDW-lattice
decoupling and the emergence of a flat band suggest that the new CDW could
arise from enhanced electron-electron interactions in the 2D limit. These
findings establish monolayer-VSe$_2$ as a host of coexisting charge orders with
distinct origins, and enable the tailoring of electronic phenomena via emergent
interactions in 2D materials.
