From 1995 to 2019, the average household debt burden in OECD countries increased from 71% to 127%. While financial deregulation and welfare retrenchment have been identified as key contributing factors, the impact of bank concentration on this trend has remained less understood. This study explores the contribution of bank concentration to household debt burden by analyzing data from the Luxembourg Wealth Study, covering over one million households across 16 affluent democracies during the same period. Results show that an increase in country-level bank concentration leads to a higher household-level debt burden. To uncover the mechanisms at play, I evaluate two hypotheses: market power and resource partitioning. Contrary to the expectations of the market power hypothesis, causal mediation analysis indicates that the relationship between bank concentration and household debt burden is not mediated by increased profitability in the banking sector. Instead, the evidence supports the resource partitioning theory, highlighting the significant role of nonbank financial organizations. Notably, the study finds that the non-housing debt burden of low-income households is particularly influenced by the proliferation of these organizations. This research sheds light on an overlooked driver of household indebtedness, illustrating an organizational process of debt-based accumulation that stratifies household wealth.