FIG. 1 is a front perspective view of a handheld ultraviolet (UV) disinfectant unit, showing my new design; FIG. 2 is top plan view of FIG. 1; FIG. 3 is a bottom plan view of FIG. 1; FIG. 4 is a right side elevational view of FIG. 1; FIG. 5 is a left side elevational view of FIG. 1; FIG. 6 is a rear elevational view of FIG. 1; and, FIG. 7 is a front elevational view of FIG. 1.