With the continually increasing operating frequencies, complex high-speed interconnect and package modules of VLSI systems require characterization based on measured/simulated scattering parameters. Several algorithms were recently suggested for macromodeling such type of data to enable unified transient analysis in the presence of external network elements. One of the critical issues involved here is the passivity violations associated with the computed macromodel. To address this issue, a new passivity check and enforcement algorithm is presented in this paper. The proposed approach significantly reduces the computational time compared to the existing techniques. Examples are presented to demonstrate the validity and efficiency of the proposed algorithm.