A payment system is any system used to settle financial transactions through the transfer of monetary value. This includes the institutions, payment instruments such as payment cards, people, rules, procedures, standards, and technologies that make its exchange possible. A payment system is an operational network which links bank accounts and provides for monetary exchange using bank deposits. Some payment systems also include credit mechanisms, which are essentially a different aspect of payment.

Payment systems are used in lieu of tendering cash in domestic and international transactions. This consists of a major service provided by banks and other financial institutions. Traditional payment systems include negotiable instruments such as drafts (e.g., cheques) and documentary credits such as letters of credit. With the advent of computers and electronic communications, many alternative electronic payment systems have emerged.

The term electronic payment refers to a payment made from one bank account to another using electronic methods and forgoing the direct intervention of bank employees. Narrowly defined electronic payment refers to e-commerce—a payment for buying and selling goods or services offered through the Internet, or broadly to any type of electronic funds transfer.

Modern payment systems use cash-substitutes as compared to traditional payment systems. This includes debit cards, credit cards, electronic funds transfers, direct credits, direct debits, internet banking, e-commerce payment systems and Buy now, pay later (BNPL).

An efficient national payment system reduces the cost of exchanging goods, services, and assets. It is indispensable to the functioning of the interbank, money, and capital markets. A weak payment system may severely drag on the stability and developmental capacity of a national economy. Such failures can result in inefficient use of financial resources, inequitable risk-sharing among agents, actual losses for participants, and loss of confidence in the financial system and in the very use of money. The technical efficiency of the payment system is important for the development of the economy.

An automated clearing house (ACH) system processes transactions in batches, storing, and transmitting them in groups. An ACH is considered a net settlement system, which means settlement may be delayed. This poses what is known as settlement risk.

Real-time gross settlement systems (RTGS) are funds transfer systems where the transfer of money or securities takes place from one bank to another on a "real-time" and on "gross" basis. Settlement in "real time" means that payment transaction does not require any waiting period. The transactions are settled as soon as they are processed. "Gross settlement" means the transaction is settled on one to one basis without bunching or netting with any other transaction. Once processed, payments are final and irrevocable.

International payment systems operate like their domestic counterparts but have additional complexity as they need to navigate currency exchange and cross-border jurisdictional and financial environments. The key system used for international transfer has been the SWIFT network that is used by banks to transfer funds between countries. International transfers have traditionally been cumbersome, error prone and expensive. Increasing globalization has driving corporations and individuals to transact more frequently across borders.

Transaction volume continue to increase with people buying from foreign eCommerce sites as well as traveling, living, and working abroad. For the payments industry, the result is higher volumes of payments—in terms of both currency value and number of transactions. This is also leading to a consequent shift downwards in the average value of these payments.

Payments systems set up decades ago might be retrofitted or force-fitted to meet modern business needs. Frequently, the systems become unstable or less reliable, for example, STEP2 (an upgrade from 2003), which processes only euros. This has led to new transfer services that use financial technology or cryptocurrency to make it easier, faster and cheaper to transfer funds internationally.

As of 2014, STEP2 is the only Pan-European automated clearing house (or PE-ACH system) in operation. This type of system is thought to become less relevant as banks will settle their transactions via multiple clearing houses rather than using one central clearing house.