The bank, which has $2 trillion in assets, posted a message on its Chase.com portal asking customers to "log on later."
The bank said its online services were lost sometime "overnight" last night, and it would not predict when it expects the site to be restored.
ATMs, branches and telephone call centers are not affected by the outage, though call center volume has increased, said Tom Kelly, a Chase spokesman.
The cause of the outage isn't clear, Kelly said, adding "it's a technical issue that we're dealing with."
Twitter postings about the outage have been fast and sometimes furious. Tweets put various time frames on the outage, some showing it may have started 18 hours or more hours ago.
Kelly couldn't pinpoint the outage start time but said he last used the site at around 8 p.m. Eastern time.
"For a bank of Chase's size to be out this long is certainly unusual -- this time of the day at least," said Emmett Higdon, an analyst at Forrester Research.
Higdon said that most such outages are caused conjunction with new rollouts or a new vendor being brought out, said Higdon.
Outages by major commercial operations aren't rare, but they're usually limited to a few hours, he added.
Customers have been so sold on the idea on online functionality that they're going to notice when you go down, said Higdon. If systems are "down for anything other than a short period of time, it would have a tremendous impact."
