A new low-power custom integrated lock-in amplifier (LIA) with semi-digital automatic phase tuning is presented. The proposed LIA leverages a new semi-digital phase tuning peripheral loop to automatically align the relative phase of a low-amplitude input signal of known carrier with that of a reference signal to increase sensitivity while decreasing the power consumption of the LIA compared with previous solutions. The proposed LIA is designed for a specific reference frequency of 1.024 KHz which is suitable for most biomedical fiber photometry applications. The proposed LIA is implemented in a 0.18-μm CMOS technology with power supply voltage of 1.8 V. The proposed LIA consumes 60 μW at an operating frequency of 1.024 KHz, and presents a dynamic reserve of 33 dB for a detection bandwidth of 100 Hz and a FOM of 1.77 nW/Hz.