Massive multiple-input multiple-output (MIMO) technology has become a cornerstone of 5G and is expected to play an even greater role in future 6G networks, offering high spectral efficiency, energy efficiency, and user capacity. However, implementing fully digital precoding in ultra-dense heterogeneous networks (HetNets) operating in the millimeter-wave (mmWave) spectrum remains impractical due to excessive hardware complexity, power consumption, and sensitivity to blockage and non-line-of-sight (NLOS) conditions. To address these challenges, this paper proposes a low-complexity hybrid analog–digital precoding scheme tailored for ultra-dense mmWave massive MIMO HetNets. The design integrates analog beamforming with a low-dimensional digital baseband precoder, leveraging a block diagonalization framework to suppress inter-user interference while reducing the number of RF chains. Simulation results demonstrate that the proposed scheme achieves up to 35% and 60% higher effective spectral efficiency compared with conventional distributed MMSE and channel inversion–based hybrid precoding. The findings highlight the potential of hybrid architectures as a practical solution for interference-free, energy-efficient communication in next-generation ultra-dense networks.