Nerdfighteria (/ˌnɜːrdfaɪˈtɛriə/ NURD-fy-TERR-ee-ə) is a mainly online-based community subculture that originated on YouTube in 2007, when the VlogBrothers (John and Hank Green) rose to prominence in the YouTube community. As their popularity grew, so did coverage on Nerdfighteria, whose followers are individually known as Nerdfighters. The term was coined when John misread the title of the arcade game Aero Fighters as Nerd Fighters.

Hank Green describes it as "a community that spr[a]ng up around our videos, and basically we just get together and try to do awesome things and have a good time and fight against world suck," which he defines as "the amount of suck in the world". The Greens established The Foundation to Decrease World Suck in order to raise funds and launch projects that would help a variety of causes. Nerdfighters believe in fighting world suck, promoting education, freedom of speech, and the use of the intellect in modern society. Nerdfighters and the Green brothers have collaborated on many projects such as the annual charity fundraiser Project for Awesome and the digital media convention VidCon. Nerdfighters have been documented by websites such as The Hollywood Reporter and The Wall Street Journal, with a following estimated to be in the millions.

Nerdfighteria is known for its online collaborative nature: forums, spinoff blogs, meet-ups, and charitable events have been spawned by its members. Instances of the community collaborating can be observed in the creation of college campus groups at universities such as the University of Maryland, Texas Christian University, the University of British Columbia, and the University of California, Los Angeles. Another Nerdfighter club was founded at Auburn University, in which the members have stated their desire to do charity work with The Humane Society and This Star Won't Go Out.

The Nerdfighter subculture's outpouring of support helped convince Penguin Books to release the novel The Fault in Our Stars five months early. Additionally, safety concerns caused by 5,000 fans crowding the stage at the 2014 Q&A tour at the Dolphin Mall caused the event to be shut down early.

"What is a Nerdfighter? A Nerdfighter is a person who instead of being made out of, like, bones and skin and tissue, is made entirely of awesome."

A prominent symbol in Nerdfighter culture is a double-handed gesture with crossed arms, with both hands in a v-symbol, similar to Star Trek's "Vulcan salute". Celebrities such as actor Benedict Cumberbatch and Olympian Jennifer Pinches have been documented demonstrating Nerdfighter hand gestures. The symbol has been referred to as a Nerdfighter "gang sign" or a "salute". Additionally, the initialism "DFTBA", standing for "Don't Forget To Be Awesome", is generally considered the motto of the community.

"World suck" is another word included in the Nerdfighter vocabulary, which refers to unfavorable things that occur in the world. To decrease "world suck", the Greens founded a charitable nonprofit organization, in which Nerdfighters collaborate to promote social activism.

Pizza John, a graphic representation of a mustachioed John accompanied by the word "Pizza", is a long-standing inside joke and symbol within Nerdfighteria. On December 1, 2009, John uploaded a video called Shaving for Pakistan in which he had a mustache for the last two seconds. Nerdfighter Valerie Barr used this image to create the original Pizza John, a monochrome image of mustachioed John on a red background with the word "Pizza" underneath in sans serif font. This image was made available as a t-shirt in 2010, and it remains DFTBA Records' bestselling shirt. Over the years, Pizza John has been remixed by community members in a variety of graphic styles and mediums. Pizzamas is an annual community holiday and charity event that celebrates Pizza John.