Recently, the appeal of 2D black phosphorus (BP) has been rising due to its unique optical and electronic properties with a tunable band gap (≈0.3–1.5 eV). While numerous research efforts have recently been devoted to nano‐ and optoelectronic applications of BP, no attention has been paid to promising medical applications. In this article, the preparation of BP‐nanodots of a few nm to <20 nm with an average diameter of ≈10 nm and height of ≈8.7 nm is reported by a modified ultrasonication‐assisted solution method. Stable formation of nontoxic phosphates and phosphonates from BP crystals with exposure in water or air is observed. As for the BP‐nanodot crystals’ stability (ionization and persistence of fluorescent intensity) in aqueous solution, after 10 d, ≈80% at 1.5 mg mL−1 are degraded (i.e., ionized) in phosphate buffered saline. They showed no or little cytotoxic cell‐viability effects in vitro involving blue‐ and green‐fluorescence cell imaging. Thus, BP‐nanodots can be considered a promising agent for drug delivery or cellular tracking systems. Black phosphorus (BP)‐nanodots are prepared using a simple ultrasonication‐assisted solution method. Compared to conventional semiconductor quantum dots, BP‐nanodots present little in vitro cytotoxicity and further blue‐ and green‐fluorescent bioimaging roles under excitations of UV and visible light, showing potential as novel drug delivery carriers in biomedical applications.