The proliferation of smartphone-based payment platforms has reshaped consumer financial behaviour across emerging markets. This study investigates the impact of digital payment systems—specifically Unified Payments Interface (UPI), mobile wallets, debit/credit cards, and internet banking—on consumer behaviour in urban and semi-urban India. Employing a descriptive-analytical design with structured questionnaire data from 120 respondents, the study evaluates six a priori hypotheses concerning the relationships among ease of use, perceived security, convenience, demographic factors, and spending behaviour. Descriptive statistics, chi-square tests, and correlation analysis were applied. Results confirm that UPI dominates the digital payments landscape (50%), that daily usage is reported by 55% of respondents, and that 50% of participants experienced increased spending post-adoption, with 55% acknowledging impulse-buying tendencies. All six null hypotheses were rejected at α = 0.05, supporting the Technology Acceptance Model (TAM) and the Pain-of-Paying framework as explanatory lenses. Security and trust remain the most significant adoption barriers, particularly among older and lower-income demographic cohorts. The paper contributes an integrated behavioural model linking digital payment adoption antecedents to downstream spending outcomes, and proposes targeted recommendations for fintech firms, financial institutions, and policymakers seeking to deepen financial inclusion through responsible digitisation.