A good few bestsellers you see in bookshop windows started their lives as fanfiction. In some cases they endured heavy rewrites, in other cases it was more a case of 'filing the serial numbers off', a process in which names and concepts are changed to avoid trademark issues. Here are a few popular novels that started off as fanfics.
Honourable Mention #1: City of Bones
There was some debate about whether the Mortal Instruments trilogy was based on Harry Potter fanfic as it bears the same name as a fic the author wrote:
Mortal Instruments is a 2004 Ron/Ginny story by Harry Potter BNF Cassandra Claire. The title was later reused for Cassandra Claire's (unrelated) profic novel series, The Mortal Instruments.
Source: https://fanlore.org/wiki/Mortal_Instruments_(Harry_Potter_story)
However, the author has said that the novels were based on original ideas and did not stem from her HP fanfic. She wrote on her Tumblr blog:
"The Mortal Instruments was not a Harry Potter fanfiction. I did write Harry Potter fanfiction, that much is absolutely true. I understand that we live in a world now where people often “file the serial numbers off” their fanfiction and publish it with the names changed."
Source: https://www.tumblr.com/cassandraclare/684425518117011456/hey-cassie-i-know-tmi-was-a-harry-potter
Honourable Mention #2: Red, White & Royal Blue
There’s a lot of talk online about how this novel is based on a fanfic about Andrew Garfield and Jesse Eisenberg (yes, the real people, not the characters they play in The Social Network).
"carry it in my heart is an Andrew Garfield/Jesse Eisenberg RPF fanfic. Summary: "How Jesse learned to stop worrying and love his costar." [...] The story begins during the production of The Social Network and follows real-world events as the two actors fall in love while filming and promoting together."
Source: https://fanlore.org/wiki/Carry_it_in_my_heart
As far as I can find, there’s been no confirmation, but here’s a little sample for your reading pleasure:
“I am not giggling,” Andrew says, “monsieur.”
“Yes you are, you’re giggling like a little gay cartoon deer,” Armie slurs.
“Je suis pas,” Andrew argues. They’ve been out with him enough times that his random bouts of drunken French don’t surprise any of them anymore. “And why are you so obsédé with me being Bambi?”
“Are you saying Bambi was gay?” Justin says. “Because I find that offensive.”
(I think Armie refers to actor Armie Hammer, and Justin refers to Justin Timberlake? They're both in the movie.)
Twilight: Gabriel's Inferno
NYT bestseller Gabriel's Inferno started as a twilight fanfic. Sylvain Reynard initially posted it as an Edward x Bella student-teacher story. It's now a film which I haven’t seen but which my friend describes as “confusingly dry the same way an old cucumber is”
Source: https://www.adweek.com/galleycat/sylvain-reynard-fan-fiction/56301
Star Wars: The Love Hypothesis
Author Ali Hazelwood says her agent discovered this novel when it was a work of Kylo Ren x Rey fanfic on AO3. Allegedly the cover is fan-art. Sounds about right but I couldn’t find a good source on that.
Source: https://www.businessinsider.com/ guides/learning/fanfiction-books?r=US&IR=T#the-love-hypothesis-by-ali-hazelwood-3
Star Wars: The Hurricane Wars
Guanzon had a pretty big following online for her Reylo (again) fanfics, which led to an agent reaching out to her about transforming her fic into an original novel. She says the final novel only retains 5-10% of the original fic.
Source: https://mashable.com/article/reylo-fanfiction-the-hurricane-wars
Sailor Moon: Cinder
Author Marissa Meyer said her novel started out as an alternate reality Sailor Moon fanfiction which is still up! I’m as surprised as you are, but I recommend reading about it on her blog (source linked below).
Source: https://www.marissameyer.com/blog/luna-v-4-2-the-origins-of-cinder/
Star Wars (Again): We'd Know By Then
Another Rey x Kylo fic! Author Kirsten Bohling originally uploaded this to AO3 but it has since been deleted. Could the Reylo fandom please share some productivity tips with the rest of us mere mortals?
Source: https://www.businessinsider.com/ guides/learning/fanfiction-books?r=US&IR=T#the-love-hypothesis-by-ali-hazelwood-3
Supernatural: Point Pleasant
Author Jen Archer Wood originally wrote her thriller as Dean x Castiel fanfic (Supernatural) before reworking it.
