Northern populations of Arctic char (Salvelinus alpinus) can be anadromous, migrating annually from the ocean to freshwater lakes and rivers in order to escape sub-zero temperatures. Such seasonal behaviour demands that these fish and their associated microbiomes adapt to changes in salinity, temperature, and other environmental challenges. We have characterized the microbial community composition of anadromous S. alpinus, netted by Inuit guides at freshwater and seawater fishing sites in the high Arctic, both under ice and in open water. An on-site lab facilitated the aseptic sampling of intestine- and skin-associated microbiomes. After shipment south, bacterial profiles were generated by DNA extraction and high-throughput sequencing of PCR-amplified 16S ribosomal RNA genes. Results show that microbial communities on the skin and intestine of Arctic char are statistically different when sampled from freshwater or saline water. This association was further tested using hierarchical Ward’s linkage clustering, which not only confirmed a salinity-linked turnover but also provided evidence of a core composition of skin and intestinal bacteria.