<p>Multiport microwave power splitters are key building blocks in high-frequency systems such as phased arrays, beamforming networks and measurement setups, but are usually designed using fixed circuit topologies that are difficult to adapt to many-port or unconventional layouts. This paper introduces a scalable inverse-design framework for multiport microwave power splitters that is directly compatible with three-dimensional printing. Here we combine gradient-based optimization with adjoint electromagnetic simulations to automatically shape a dielectric device that meets specified waveform targets at multiple output ports. The method is demonstrated on a four-port power splitter operating at ten gigahertz, fabricated using a polymer powder bed fusion process (multi jet fusion) with simple constraints on minimum feature size and material permittivity. Numerical simulations and waveguide measurements show close agreement in transmission, reflection, and port-to-port balance, indicating robust performance despite manufacturing tolerances. The approach is topology-agnostic and fabrication-aware, enabling economical prototypes and systematic scaling to devices with many ports. This work establishes a general route for integrating inverse design and three-dimensional printing in microwave engineering, and could be extended to other radio-frequency and millimetre-wave components.</p>