The development of new NIR-II fluorophores, particularly those with facile syntheses, high fluorescence quantum yields, and stable and tunable photophysical properties, is challenging. Herein, we report a new class of small molecular NIR-II fluorophores based on aza-dipyrromethene boron difluoride (aza-BODIPY) dyes. We demonstrate promising photophysical properties of these dyes, such as large Stokes shift, superior photostability, and good fluorescence brightness as nanoparticles in aqueous solution. Because of these properties and high resolution and deep penetration NIR-II imaging ability, the aza-BODIPY based dyes show great potential as NIR-II imaging agents.