This work presents a DD -band multi-input multi-output (MIMO) transmitter (TX) array with a four-element joint static/dynamic beamformer. The proposed beamformer combines a static beamformer and a space-time modulated dynamic array to produce a large number of concurrent beams with fewer array elements and multiple independent data streams, essentially achieving RF-domain beam number multiplication. Consequently, the beamforming system only requires low-overhead RF/analog chains and simple digital computation, making it conducive to low-cost applications. More importantly, the concurrent multiple beams are formed at distinct carrier frequencies at various spatial angles. This allows easy discrimination of adjacent beams, which is essential for MIMO communication for high link throughput as well as MIMO radar operations for rapid situational awareness sensing of surroundings and one-shot localization. Traditional NN -element analog beamforming arrays require an MM set of independent beamformers to generate MM independent beams. The proposed NN -element beamformers can generate up to M×(N+1)M×(N+1) concurrent beams by cascading MM -beam beamformers with N+1N+1 time-modulated array (TMA) operation, which offers a new operation freedom to trade off among number of concurrent beams, array gain, spectral efficiency, and beam angle dependency. As a proof of concept, a DD -band 125-GHz four-element TX array is implemented in a GlobalFoundaries 22-nm CMOS FD-SOI process with a flip-chip package. With the beam-multiplication effect, the four-element TX array radiates a total of concurrent 20 beams with a unique space-to-frequency mapping over ±± 60 ∘∘ total beam field-of-view (FoV). Through over-the-air measurement, the TX array demonstrates concurrent four beams with 4-Gb/s 16-QAM modulation and reconfigurable space-to-frequency mapped 20 beams, each with 100-MHz bandwidth frequency-modulated continuous-wave (FMCW) chirping to support joint MIMO communication and sensing.