Knockdown or knock-down may refer to:

• Knockdown, a situation in full-contact combat sports where a fighter is down or vulnerable, often preliminary to a knockout
• "Knockdown" (Castle), the thirteenth episode of the third season of the TV series Castle
• knock-down fastener
• knock-down furniture
• Knockdown (arcade game)', an arcade game released by Namco
• Knockdown (G.I. Joe), a fictional character in the G.I. Joe universe
• "Knockdown" (song), a song by Alesha Dixon
• KnocDown, a music producer
• Knockdown, a nautical term for a near-capsize
• Knock-down kit, a complete kit needed to assemble a vehicle
• Knockdown resistance, genetic resistance to pyrethroid insecticides in many insect species
• Knockdown texture, a drywall finishing style
• Gene knockdown, a genetic modification technique