This study examines the structural reconfiguration of the Port of Dar es Salaam’s regional hinterland between 2016/17 and 2025/26 financial years, using ten consecutive years of country-level transit cargo data (in metric tons). Applying market concentration metrics, the Herfindahl-Hirschman Index (HHI) and the two-firm concentration ratio (CR2), alongside share and compound annual growth rate (CAGR) analysis, the paper tests whether the port’s hinterland has become more or less concentrated as total transit throughput roughly tripled from 4.71 million to 14.75 million tons. The findings reveal a pronounced structural shift: the Democratic Republic of the Congo (DRC) displaced Zambia as the port’s dominant transit client, its share rising from 24.2% to 52.6% while Zambia’s fell from 36.2% to 23.1%. HHI values rose steadily from 2,467 to 3,565 and CR2 from 60.4% to 75.8%, indicating that Dar es Salaam’s hinterland is transforming from a moderately concentrated to a highly concentrated market dominated by two copper-corridor economies. The paper links this reconfiguration to mining-led demand from the Congolese and Zambian Copperbelt, rail and road corridor investments, and intensifying competition with the Mombasa-based Northern Corridor. It argues that rising concentration, while evidence of Dar es Salaam’s competitive success, creates vulnerabilities that warrant deliberate diversification of transit clients and corridor capacity. The paper discusses implications for port governance, corridor policy, and regional integration.