William Henry Green II (born May 5, 1980) is an American YouTuber, science communicator, novelist, stand-up comedian, and entrepreneur. He produces the YouTube channel Vlogbrothers with his older brother, author John Green, and hosts the educational YouTube channels Crash Course and SciShow. He has advocated for and organized social activism, created and hosted a number of other YouTube channels and podcasts, released music albums, and amassed a large following on TikTok.

With his brother John, Hank co-created VidCon, the world's largest conference about online videos, and the Project for Awesome, an annual online charity event, as well as the now-defunct conferences NerdCon: Stories, focused on storytelling, and PodCon, focused on podcasts. He is the co-creator of The Lizzie Bennet Diaries (2012–2013), an adaptation of Pride and Prejudice in the style of video blogs that was the first web series to win an Emmy. He is also the co-founder of merchandise company DFTBA Records, crowdfunding platform Subbable (acquired by Patreon), game company DFTBA Games, and online video production company Pemberley Digital, which produces video blog adaptations of classic novels in the public domain. Green is the founder of the environmental technology blog EcoGeek, which evolved into Complexly, an online video and audio production company of which he was the CEO until late 2023. Green also hosts the podcasts Dear Hank & John and Delete This with his brother and wife respectively, along with the podcast SciShow Tangents.

Green's debut novel, An Absolutely Remarkable Thing, was published on September 25, 2018; its sequel A Beautifully Foolish Endeavor was published on July 7, 2020. Both novels debuted as New York Times Best Sellers. In response to being diagnosed and treated for Hodgkin lymphoma in 2023, Green stepped down as CEO of his companies. While recovering, Green began performing stand-up about his experience. His comedy special titled Pissing Out Cancer was released on the streaming service Dropout on June 21, 2024.