This article presents the novel design techniques for terahertz (THz) antenna-in-package (AiP) to overcome issues resulting from multilayer printed circuit board (PCB)-based antenna array integration, fabrication, and measurement. We propose a wideband dual-polarized stub-loaded proximity-coupled stacked patch antenna and vertical power divider as design solutions. Circuit analysis is performed to provide the equivalent circuit model and its design validity. We designed, simulated, and measured a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4 \times 1$ </tex-math></inline-formula> subarray, which ultimately builds THz <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8 \times 8$ </tex-math></inline-formula> AiP for the next generation 6G communications system. The designed proximity-coupled stacked patch antenna employs open and short stubs in via transition of the antenna’s feeding for bandwidth enhancement. The designed power divider is constructed vertically to minimize the size of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4 \times 1$ </tex-math></inline-formula> subarray. This innovative power-splitting architecture significantly decreases the complexity of the feeding network because the divider is designed without conventional T-junctions and quarter wavelength impedance transformers (ITs); thus, the size of the dual-fed dual-polarized <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4 \times 1$ </tex-math></inline-formula> subarray, limited by two four-way power dividers can be shrunken considerably. Based on these proposed techniques, we fabricated and measured an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8 \times 8$ </tex-math></inline-formula> antenna array package on our custom-built probe station. The measured boresight gain of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8 \times 8$ </tex-math></inline-formula> antenna array package exceeds 17.1 dBi within the operating bandwidth of 136–148 GHz.