Coffee waste-derived carbon dots (C-CDs) that emit blue light were synthesized via hydrothermal process. The synthesized C-CDs have a round shape with average diameter of ~3.7 nm. The C-CDs show PL emission centered at 450 nm with excitation wavelength at 360 nm. The C-CDs show promising applications as Fe3+ sensors in aqueous solutions by fluorescence quenching. The C-CDs exhibited strong turn-off fluorescence when trace Fe3+ was added to the solution. Furthermore, the C-CDs-Fe3+ can also be used as a turn-on sensor for the detection of ascorbic acid (AA). AA reduces Fe3+ to Fe2+, resulting in the recovery of fluorescence intensity of the quenched C-CDs. Thus, the C-CDs can be used as a rapid and effective dual mode “off-on” fluorescence sensor for Fe3+ and AA. The fluorescence response exhibits a good linear relationship with the concentration of Fe+3 and AA in the range of 0-100 and 0- 1 μM, with a limit of detection (LOD) around 4.314 and 0.162 μM, respectively.