Femslash (also known as "f/f slash", "femmeslash", "altfic" and "saffic") is a subgenre of slash fan fiction which focuses on romantic and/or sexual relationships between female fictional characters. Typically, characters featured in femslash are heterosexual in the canon universe; however, similar fan fiction about lesbian characters is commonly labeled as femslash for convenience. The term is generally applied only to fanworks based on Western fandoms; the nearest anime/manga equivalents are more often called yuri and shōjo-ai fanfiction. "Saffic" is a portmanteau of Sapphic and fiction. "Altfic" as a term for fanfiction about loving relationships between women was popularized by Xena fans.

As of 2006, femslash is enjoying increasing popularity and is the "dominant form" of slash in some fandoms.

There is less femslash than there is slash based on male couples; for example, in the Lord of the Rings fandom, only a small number of femslash stories are written about the Arwen/Éowyn pairing in comparison to slash between the male characters. It has been suggested that heterosexual female slash authors generally do not write femslash, and that it is rare to find a fandom with two sufficiently engaging female characters. Janeway/Seven is the main Star Trek femslash pairing, as only they have "an on-screen relationship fraught with deep emotional connection and conflict". Although it is debated whether fanfiction about canon lesbians such as Willow and Tara of Buffy the Vampire Slayer counts as "slash", their relationship storylines are more coy than heterosexual ones, which entices Willow/Tara femslash authors to fill in the gaps in the known relationship storyline. It is "relatively recently" that male writers have begun writing femslash, and this entry of males into femslash has occurred within Buffy femslash. The femslash authorship is mostly female.