This paper analyzes the expansion of the Unified Payments Interface (UPI) within India’s digitized payment ecosystem from April 2016 to December 2021 using secondary data, official data, and contemporary literature. Results indicate that UPI grew from a modest payment interface system to a substantial payment infrastructure system. The number of monthly transactions surged from 0.09 million in July 2016 to 4,566.30 million in December 2021, and the number of banks on the system grew from 21 to 282. According to official data, in the fiscal year 2025–26, UPI accounted for 24,161.69 crore transactions valued at ₹ 314 lakh crore and was responsible for approximately 85% of digital transactions in India. This study states that UPI’s boom can be attributed to a combination of factors that include interoperability, mobile access, QR-based payments, low cost, direct bank access, public-private collaboration, and network effects. For sustained growth, UPI must address fraud, ensure access and resilience, intensify competition, and establish the system with sustainable network effects.