Yarn bombing (or yarnbombing) is a type of graffiti or street art that employs colorful displays of knitted or crocheted yarn or fibre rather than paint or chalk. It is also called wool bombing, yarn storming, guerrilla knitting, kniffiti, urban knitting, or graffiti knitting.

While other forms of graffiti may be expressive, decorative, territorial, socio-political commentary, advertising or vandalism, yarn bombing was initially almost exclusively about reclaiming and personalizing sterile or cold public places. It has since developed with groups graffiti knitting and crocheting worldwide, each with their own agendas and public graffiti knitting projects being run.

According to Manuela Farinosi and Leopoldina Fortunati, yarn bombing has become synonymous with the current feminist movement due in part to the reclamation of the traditionally feminine arts of knitting and crocheting to partake in the traditionally masculine and male-dominated graffiti scene. The women and girls who make up the yarn bombing subculture are diverse in race, age, sexuality, class, etc., and create space for themselves and their art everywhere from college campuses to public parks. This creation and preservation of space is what motivates some of the participants, some of whom have never been able to access a political art space before. In her article about yarn bombing, Joanna Mann explains the balance between the art and politics, "Yarn bombing, I argue, does more than feminise the city, for the whimsy with which it is imbued has the capacity to increase our attentiveness to habitual worlds in a series of micro-political gestures." McGovern finds that yarn bombing may also be used to contradict the idea of women as homemaker by bringing such traditionally feminine art into public space.

The practice is believed to have originated in the U.S. with Texas knitters trying to find a creative way to use their leftover and unfinished knitting projects, but it has since spread worldwide.

The start of this movement has been attributed to Magda Sayeg, from Houston, who says she first got the idea in 2005 when she covered the door handle of her boutique with a custom-made cozy.

Houston artist Bill Davenport was creating and exhibiting crochet-covered objects in Houston in the 1990s, and the Houston Press stated that "Bill Davenport could be called the grand old man of Houston crocheted sculpture." Artist Shanon Schollian was knitting stump cozies in 2002 for clear cuts in Oregon. The Knit Knot Tree by the Jafagirls in Yellow Springs, Ohio, gained international attention in 2008.

The movement moved on from simple 'cozies' with the innovation of the 'stitched story'. The concept has been attributed to Lauren O'Farrell (who creates her street art under the graffiti knitting name Deadly Knitshade), from London, UK, who founded the city's first graffiti knitting collective Knit the City. The 'stitched story concept' uses handmade amigurumi creatures, characters, and items to tell a narrative or show a theme. This was first recorded with the Knit the City collective's "Web of Woe" installation in August 2009.

The Knit the City collective were also the first to use O'Farrell's term yarnstorming to describe their graffiti knitting, as an alternative to the more popular term yarnbombing.