Inverse design has emerged as a powerful approach for developing high-performance photonic devices beyond the limitations of conventional geometry-based methods. In this study, we propose a compact 3 dB optical power splitter designed using a boundary-based inverse design strategy driven by the particle swarm optimisation (PSO) algorithm. The device boundary along the propagation direction is discretised into fine segments and iteratively optimised through a two-stage framework to simultaneously suppress input reflection and achieve balanced power distribution at the output ports. Over a 100 nm bandwidth, the improved structure exhibits outstanding optical performance. The excess loss stays between −0.1 and −1.1 dB, but the reflection is decreased to about −20 dB. Nearly equal power splitting is seen at the middle wavelength when the balancing factor is close to 0 dB. The device also exhibits symmetric and consistent responses when excited from either input port. With a compact footprint of approximately 6 µm × 16 µm, the proposed design is suitable for high-density photonic integrated circuits. These results confirm the effectiveness of a PSO-based boundary inverse design for realising broadband, low-loss, and compact photonic components.