This letter presents a 28 GHz 2-dimensional (2-D) multi-beam array antenna fed by a planar 4 × 4 cascaded Butler matrix (CBM). Utilizing the proposed concept of 3—D to planar CBM topology transition, the 4 × 4 U-slot antenna array integrated with planar CBM can be implemented with a multi-layers lamination without additional connectors or routing lines. Due to the simplified topology of the circular sub-BM, whose width is only 0.5 $\lambda_0$ (free space wavelength at 28 GHz), none of the crossovers is needed to assemble the radiation elements at the outputs of the CBM. The dimension of the proposed 2-D beam steering system has been dramatically reduced to $3.3 \ \lambda_0 \times 5.9 \ \lambda_0$ , which is, to best of our knowledge, the smallest in this application area. A prototype has been fabricated and measured. The measured results show a well-controlled input matching in the operation band from 26 GHz to 29 GHz. Besides, 16 spatial orthogonal beams with maximal 15.22 dBi gain at 28 GHz scanning a conical space with the maximal cone angle of 77.4° in the elevation plane and 136.8° in the azimuth plane by feeding at different inputs, which agree well with the simulated results.