The breach came after the laptop of an employee at Hewlett Packard Enterprise working on a naval contract was "compromised", the Navy said.
It added that "unknown individuals" accessed the sensitive information on current and former sailors.
The data included names and social security numbers, but the Navy does not currently believe it was misused.
"The Navy takes this incident extremely seriously - this is a matter of trust for our sailors," said Vice Adm Robert Burke, the chief of naval personnel.
"We are in the early stages of investigating and are working quickly to identify and take care of those affected by this breach."
Laptop breach
The US Navy has about 430,000 sailors on active duty or in ready reserve.
Sailors are being contacted in the coming weeks and the Navy said it was looking into credit monitoring services for those affected.
Hewlett Packard Enterprise, which separated from US computer firm HP last year, informed the Navy on 27 October about the laptop.
After a Naval Criminal Investigative Service (NCIS) investigation, it was found that the data of 134,386 sailors had been accessed.
A spokesman for Hewlett Packard Enterprise said: "This event has been reported to the Navy and because this is an ongoing investigation, HPE will not be commenting further out of respect for the privacy of our Navy personnel."
It added that "unknown individuals" accessed the sensitive information on current and former sailors.
The data included names and social security numbers, but the Navy does not currently believe it was misused.
"The Navy takes this incident extremely seriously - this is a matter of trust for our sailors," said Vice Adm Robert Burke, the chief of naval personnel.
"We are in the early stages of investigating and are working quickly to identify and take care of those affected by this breach."
Laptop breach
The US Navy has about 430,000 sailors on active duty or in ready reserve.
Sailors are being contacted in the coming weeks and the Navy said it was looking into credit monitoring services for those affected.
Hewlett Packard Enterprise, which separated from US computer firm HP last year, informed the Navy on 27 October about the laptop.
After a Naval Criminal Investigative Service (NCIS) investigation, it was found that the data of 134,386 sailors had been accessed.
A spokesman for Hewlett Packard Enterprise said: "This event has been reported to the Navy and because this is an ongoing investigation, HPE will not be commenting further out of respect for the privacy of our Navy personnel."
