A grey hat (greyhat or gray hat) is a term used in computer security with a range of definitions and is derived from the concepts of "white hat" and "black hat" hackers.

The term was first used in print in the late 1990s by the hacker group L0pht in a 1999 interview with The New York Times to describe their hacking activities. The same article describes several members as black hat hackers and a third party describes L0pht as white-hat hackers.

Another definition of grey hat maintains that grey hat hackers only arguably violate the law in an effort to research and improve security: legality being set according to the particular ramifications of any hacks they participate in.

In the search engine optimization (SEO) community, grey hat hackers are those who manipulate websites' search engine rankings using improper or unethical means but that are not considered search engine spam.

A 2021 research study looked into the psychological characteristics of individuals that participate in hacking in the workforce. The findings indicate that grey hat hackers typically go against authority, black hat hackers have a strong tendency toward thrill-seeking, and white hat hackers often exhibit narcissistic traits.

In April 2000, hackers known as "{}" and "Hardbeat" gained unauthorized access to Apache.org. They chose to alert Apache crew of the problems rather than try to damage the Apache.org servers.

In August 2013, Khalil Shreateh, an unemployed computer security researcher, exploited a Facebook vulnerability to post a message on the wall of founder Mark Zuckerberg after the company's bug bounty program had dismissed his report. Facebook fixed the vulnerability but refused to pay Shreateh a bounty, saying his demonstration on a real account had violated its responsible disclosure terms; the case was widely reported as an example of grey hat disclosure.