A custom tranceiver IC is designed to implement a portable brain imaging system based on functional near-infrared spectroscopy (fNIRS). The fNIRS IC generates multichannel time-divided spread-spectrum codes (TDSSCs) and drives light-emitting devices in the transmitter (Tx) chain, and performs optimum filtering, quantization, and serialization in the receiver (Rx) chain. A dual slope ADC subsequent to an operational transconductance amplifier-C-based matched filter shares a capacitor to save area while achieving optimum signal-to-noise ratio (SNR) in the brain channel. The Rx chain including an off-chip TIA ensures sufficient electrical SNR irrespective of the brain region for the accurate extraction of hemodynamic response. The minimum detectable light power of the Rx chain is 400 fW. The output power of the Tx is made adjustable by controlling the occurrence rate and the power of the TDSSC to reduce subject-dependent measurement variations. The proposed fNIRS system measures 42 brain regions simultaneously, and the experimental results clearly verify the validity of the proposed portable fNIRS system.