Explaining the modern cultural and linguistic diversity present on the Sepik coast requires an understanding of long-term interaction on both a regional and a broader Melanesian scale. To assess the nature of prehistoric social networks, 438 obsidian specimens and 326 ceramic sherds collected from the coast and brought to the Field Museum of Natural History in Chicago were subjected to chemical analysis by either laser ablation inductively coupled plasma mass spectrometry or portable X-ray fluorescence. The results indicate that people living on the Sepik coast received obsidian from sources in the Admiralty group and on New Britain continuously for the past 2,000 years, and possibly as far back as the mid-Holocene. While obsidian from New Britain is more abundant in contexts believed to pre-date ∼2,000 BP, the more proximal Admiralty sources dominate later assemblages. Ceramic exchange may have begun between production centers on the coast as early as 2,000 years ago, and spanned the length of the coast by at the latest 1,000 BP. Regional differences in the degree and scope of exchange relationships evident in the recent past may have very ancient roots—the present data suggest that social networks on the coast were comparable to those ethnographically documented, and that sometimes quite proximal sites obtained materials from different sources or networks of exchange partners.