An array substrate for a digital X-ray detector, a digital X-ray detector including the same, and a method for manufacturing the same are disclosed. The array substrate reduces a step difference of a PIN diode, removes a bent part from a lower part to reduce characteristic deterioration of the PIN diode, and increases the size of a formation region of the PIN diode to increase a fill factor. To this end, the array substrate allows a source region of an active layer included in a thin film transistor to be in surface contact with a lower electrode of the PIN diode, and disposes the lower electrode over a planarized source region or a base substrate, such that a step difference of the PIN diode is reduced and fill factor is improved.