In this paper, an ultrathin and ultrabroadband metamaterial absorber based on 3D water microchannels is proposed. The experimental results show an absorption rate over 90% and a relative bandwidth up to 165% in the frequency band between 9.6 and 98.9 GHz. This polarization-independent absorber can work at a wide angle of incidence and exhibits good thermal stability. Benefiting from ultrabroadband absorption, thin thickness, low cost, and environmentally friendly materials, the proposed metamaterial absorber can be used in the fields of electromagnetic wave stealth and electromagnetic radiation protection. Related device design and research methods can be extended to the applied research in the terahertz and optical bands.