The region's central bank said Tuesday that overnight deposits from Monday hit €481.93 billion.
That exceeded the old record of €463.56 billion from the day before.
The high deposits mean banks are keeping spare cash in a safe place at a low interest rate rather than lending it one another on a short-term basis. This move has sparked fears of a further credit crunch as banks become wary about lending funds.
The deposits also reflect large amounts of cash put into the banking system from ECB emergency loans of €489 billion taken up by more than 500 banks in late December.