This article introduces a four-element 300-GHz-band bi-directional phased-array transceiver (TRX). The TRX utilizes the same antenna, signal path, and local oscillator (LO) circuitry to operate either in transmitter (TX) mode or receiver (RX) mode. The TX mode adopts the outphasing technique to increase the average output power for higher order modulation schemes by utilizing the two mixers that are connected directly to the antenna in a mixer-last fashion. The two signal paths also enable the canceling of the LO feed-through (LOFT). The RX mode also benefits from the LOFT cancellation technique to suppress the LO emission, which is a common issue of the mixer-first RXs. The RX has a separate Hartley operation mode to reject the image signal coming from the TX. The TRX chip was implemented using CMOS 65-nm process, and a four-element phased array was implemented by stacking liquid crystal polymer (LCP) flexible printed circuit boards (PCBs). The stacked structure provides the required narrow antenna pitch at the 300-GHz band. The measured beam angle range is from <inline-formula> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula>18<inline-formula> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> to 18<inline-formula> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula>. The single-element power consumption is 750 mW for both TX mode and RX mode.