A multiphysics-based co-simulation technique is developed based on the finite element method for the characterization of 3-D high-frequency integrated circuits (ICs) with integrated microchannel cooling. The multiphysics in the cosimulation includes full-wave electromagnetic, fluid, and transient conjugate heat transfer analyses. For incompressible and fully developed flows, the governing equations of fluid flow are decoupled from temperature, and the velocity field is obtained analytically. The full-wave electromagnetic and the transient conjugate heat transfer analyses are coupled through temperature-dependent material properties. The efficiency of the co-simulation is enhanced through an adaptive time-stepping scheme, a domain decomposition scheme called the finite element tearing and interconnecting (FETI), and FETI-enabled parallel computing. The capability and the efficiency of the proposed co-simulation in analyzing practical and large-scale 3-D ICs are demonstrated through numerical examples. The effectiveness of heat removal through integrated microchannel cooling and the thermal impact on high-frequency characteristics of 3-D IC designs are also demonstrated.