Electronic services of many banks - ATMs, mobile and internet banking - crashed on Friday morning. Messages on some screens said there were too many users.
One big bank was later disconnected from the central switching system and other banks' services were restored.
Users complained about the problem through many media, as it is salary payment day for many people. Some commercial banks announced they were aware of the problem and promised to restore services as soon as possible.
Complainants referred to problems with Kasikornbank, Krungthai Bank, Thanachart Bank and Krungsri Bank.
Kasikornbank posted a message on its Facebook page KBank Live, saying parts of its system broke down and customers were unable to make some transactions, especially inter-bank ones.
Krungsri Bank stated on its Facebook page that its Krungsri Mobile Application was out of use, but customers could use online and ATM services.
Krungthai Bank stated on the homepage of its internet banking service that "Due to high volumes of transactions, our website is currently unavailable. We apologize for the inconvenience. Please try again later."
Thai News Agency quoted Yos Kimsawatde, head of the payment system office of the Thai Bankers Association, as saying the problem started with a malfunctioning at Kasikornbank, which affected other banks because they shared a central switching system.
Kasikornbank was considering shutting down its system temporarily for checks and tests, he said.
Ronadol Numnonda, Bank of Thailand assistant governor (supervision), said later that a big bank was overwhelmed with backlogs of inter-banking transaction requests, which affected transactions with many other banks, especially those through mobile banking.
As earlier agreed, the BoT excluded the bank from the central system, and most banking systems were restored, he said.
Mr Ronadol did not say which bank.