The Unified Payments Interface (UPI), launched by the National Payments Corporation of India (NPCI) on April 11, 2016, has emerged as the world's largest real-time digital payments platform. This paper examines UPI across three analytical dimensions: its historical evolution in the context of global digital payment development; a structured comparison of UPI against the world's five leading payment systems — Alipay, WeChat Pay, PayPal, Apple Pay, and Zelle; and an intrastate analysis of UPI adoption patterns across Indian states and Union Territories as of December 2025. Using secondary data from NPCI, the Reserve Bank of India (RBI), the World Bank, and peer-reviewed literature published up to December 2025, the study applies a descriptive-comparative research design.