Co-presidents Daniel Pinto and Gordon Smith sent a memo to employees and shareholders which said Dimon underwent successful emergency heart surgery and was alert and doing well: 

"We want to let you know that Jamie experienced an acute aortic dissection this morning. He underwent successful emergency heart surgery to repair the dissection. The good news is that it was caught early and the surgery was successful. He is awake, alert and recovering well."

CNBC's Wilfred Frost reported Dimon woke up with chest pains Thursday morning and went to the hospital.

Pinto and Smith will be jointly leading the company as Dimon, 63, recuperates, the memo said.

"As Co-Presidents and Chief Operating Officers, we have been working hand-in-hand with Jamie and the Board over the past two years to help lead our company," Pinto and Smith said in the memo. 

An aortic dissection occurs when the inner layer of the large blood vessel branching off the heart tears, according to the Mayo Clinic, which says the condition can be fatal.

The bank's lead director Lee Raymond said in the memo that the board had been "fully briefed on these developments and has asked Daniel and Gordon to lead the company during this period."

Raymond added that the company has "exceptional leaders across our businesses and functions – led by our outstanding CEO and Co-Presidents. Our company will move forward together with confidence as we continue to serve our customers, clients, communities and shareholders."