I read a LOT of fanfiction in English, and I was wondering if a Japanese equivalent to fanfiction.net exists. I tried searching for　ファンフィクション but didn't really get any solid results. Different terminology?

look up 同人誌 (どうじんし).

~Slaps forehead~

Well that was obvious. I'll check it out, thanks. I was under the impression that 同人誌 mostly covered fan-made manga and not literature, but I'll see.

Usually I find it under "小説" or "Novel". There isn't really a fanfiction.net type site in Japanese. Mostly people make personal sites, and host stuff like fanfiction and fanart on them. I would find a search portal (サーチ　or ナビ） for your favorite series (e.g here's one for Yuu Yuu Hakusho), you'll probably have more luck with more popular series. Once you find one, click on your favorite character, or pairing (or just 小説), to find stuff.

Hahah, funny I de-lurk on this topic, but I've been getting quite a bit of reading practice using fanfics lately... Like Bokusenou mentioned, searching for whatever name of fandom plus + 小説 usually works for me, but that can be broad and get a lot of extraneous results. 二次創作(物) is the technical term for derivative works, but I almost always see people just refer to fics as 小説.

2.novelist.jp is a big multi-fandom fanfic archive. Unlike FF.net, it uses tags instead of categories, and I think you have to register for an account to read "mature" works.

Also, pixiv is a popular site that features a lot of fanart. However, if you click on a tag for whatever fandom you're interested in, you can toggle between art [イラスト・漫画] and fanfiction [小説] using a button in the top-right corner.

That's EXACTLY what I was looking for. Thank you!