Antenna modules employing three-dimensional (3D) build-up on mold package to support efficient integration of radio-frequency (RF) circuitry, and related fabrication methods. The antenna module includes a RF transceiver whose circuitry is split over multiple semiconductor dies (“dies”) so different semiconductor devices can be formed in different semiconductor structures. The antenna module is provided as a 3D build-up on mold package to reduce lengths of die-to-die (D2D) interconnections between circuits in different dies. First and second die packages that include respective first and second dies encapsulated in respective first and second mold layers are coupled to each other in a vertical direction in a 3D stacked arrangement with active faces of the first and second dies facing each other to provide a reduced distance between the active faces of the first and second dies. An antenna is stacked on the second die package to provide an antenna(s) for the antenna module.