An adjustable digital lock detector for a phase-locked loop (PLL) has a variable counter for outputting an output signal corresponding to a first clock signal, a target count number signal, and a count number offset signal, a latch for sampling the output signal of the variable counter and outputting a latch output signal according to a result of sampling the output signal, a lead/lag detector for receiving the latch output signal and outputting the count number offset signal according to a predetermined state of the latch output signal, and an arbiter for receiving the latch output signal and outputting an arbiter output signal according to the latch output signal and a second clock signal.