Melanoma is a type of highly malignant and metastatic skin cancer, and early detection of melanoma by analyzing the level of its biomarker may decrease the likelihood of mortality. In this study, a fluorescent probe called NBR-AP for detecting tyrosinase (a biomarker of melanoma) has been created by incorporating a hydroxyphenylurea group (as a substrate for the enzyme) onto a fluorescent dye phenoxazine derivative (as an activatable signal reporter). This probe can be activated to generate fluorescence through a tyrosinase-mediated oxidation followed by hydrolysis of the urea linkage. The probe is able to detect the endogenous tyrosinase level in live cells and in zebrafish sensitively and selectively. Moreover, by imaging the tyrosinase activity, NBR-AP has been successfully applied to diagnose the melanoma and its metastasis in xenogeneic mouse models established via subcutaneous injection of B16F10 cells.