Influenza A(H3N2) subclade K virus was detected in Canada early in the 2025/26 influenza season, bearing an antigenic transition in the hemagglutinin (HA) glycoprotein. Analysis of 396 HA sequences from Canada showed antigenic divergence from 2025/26 influenza vaccine strains, consistent with partial mismatch. Phylodynamic analysis revealed sustained pre-vaccine transmission without clear post-vaccine expansion. Phylogenetic and phylogeographic analyses indicated interprovincial mixing within a highly connected metapopulation, highlighting the value of genomic surveillance for real-time epidemiologic inference and public health decision-making.