The invention relates to a diesel fuel filter for a vehicle. The diesel fuel filter includes: a filter assembly for removing foreign substances or water contained in fuel; a lower cap, through which water is discharged, the lower cap being connected to a lower discharge part of the filter assembly; a double-lock cap, which is engaged with the lower cap through a gear structure to prevent the lower cap from loosening, the double-lock cap being connected to the lower discharge part of the filter assembly; and a double-lock pin, which locks the double-lock cap and is connected to the double-lock cap and the lower discharge part of the filter assembly. Therefore, fuel leakage caused by unfastening of the lower cap due to impact or vibration during driving of a vehicle is prevented.