The extensive list of users' security details is thought to have been sourced from a cyber-attack on the business networking site in 2012.
According to the news site Motherboard, a hacker called "Peace" is selling the data on The Real Deal, a dark web illegal marketplace, for five bitcoins £1,500 .
Four years ago LinkedIn admitted to a security breach but said only 6.5 million users' account details were posted online.
"Yesterday, we became aware of an additional set of data that had just been released that claims to be email and hashed password combinations of more than 100 million LinkedIn members from that same theft in 2012," the company's chief information security officer Cory Scott wrote in a blog post.
For LinkedIn, the lesson is the same as four years ago: "don't store passwords in an insecure way", says Motherboard.
News of the breach is the latest in a long line of cyber-attacks on major websites and companies, with telecoms firm TalkTalk and parental forum Mumsnet among those who have fallen victim to security breaches in the last year.