Janus particles with controlled asymmetries and functionalities represent promising building blocks and func-tional materials due to their unique anisotropic features. Herein, we show a facile and general approach towardspreparing colloidal Janus particles with adjustable surface asymmetries and functionalities based on monolayercolloidal crystal (MCC) templates combined with mussel-inspired polydopamine (PDA) chemistry at an air-waterinterface. First, monodisperse colloidal polystyrene/polydopamine (PS/PDA) Janus particles with tunableasymmetrical geometries are synthesized by polymerizing dopamine onto two-dimensional (2D) polystyrene(PS) monolayer colloidal crystals formed at the air-water interface. Moreover, the good chemical reactivity of thePDA coating makes it a versatile platform to introduce a variety of functional materials, e.g., metal nanoparticlesand organic molecules, producing colloidal Janus particles with adjustable functionalities. Finally, we demon-strate this synthetic strategy not only provides a controllable method for the fabrication of monodisperse col-loidal Janus particles but also opens up a new Janus platform for artificially designed Janus particles withtunable asymmetries and functionalities.