This book constitutes the proceedings of the 27th International Conference on Passive and Active Measurement, PAM 2026, held as a virtual event during March 23-25, 2026. The 9 full papers and 9 short papers included in the book were carefully reviewed and selected from 51 submissions. They were organized in topical sections as follows: Routing; Topology; Applications and Congestion; DNS; Wireless and Mobile; Security and Privacy; and Virtualization.