Daniel John Hardcastle (born 23 March 1989), known online as Nerd³, is a British author and YouTuber.

Hardcastle first came to prominence through a scripted, story-based webcomic made within the game Minecraft. After creating his YouTube channel on 20 March 2011, Hardcastle initially began uploading a series of comedic vlogs before uploading his first let's play - the start of a Minecraft series called "Nerd³'s Minecraft Buildy Thing" - on 27 August. After creating videos on a few indie games, Hardcastle uploaded his first "Nerd³ Plays" video, on Battlefield 3, on 7 January 2012. In 2012 Machinma sponsored him to create the series 13 Ways To Die. He has collaborated with Emma Blackery, Tomska, Hannah Hart, Stuart Ashen and InTheLittleWood.

Hardcastle began writing his first book, Fuck Yeah, Video Games: The Life & Extra Lives of a Professional Nerd, in 2017. Crowd-funded through the website Unbound, the book reached 100% of its target on the first day (4 June 2018)^[1], and 1000% by 11 June. Reaching a total of 1897% of its initial goal, Fuck Yeah, Video Games is the most successful crowd-funded book of all-time. The foreword of the book was written by Tim Schafer^[2]. and also featured a chapter written by Stuart Ashen. It was released on 19 September 2019 and was a Sunday Times best seller. On 26 September 2019 crowdfunding began for Hardcastle's second book, The Paradox Paradox.

Hardcastle is married to Rebecca Maughan^[3], an artist who illustrated Fuck Yeah, Video Games. Hardcastle's father, Steve "Dad³" Harcastle, also started a YouTube channel in 2013 which has over 300,000 subscribers and 23 million views.

Hardcastle studied astrophysics at the University of Leicester but dropped out after the first year.