K9

K9 or K-9 most commonly refers to:
 * K9, the nickname of police dogs and the police dog unit itself
 * Canine or Canis, a genus including dogs, wolves, coyotes, and jackals

K9 or K-9 may also refer to:

Fictional dogs

 * K9 (Doctor Who), the name of several robotic canines in the British science fiction television series Doctor Who
 * K-9 (Looney Tunes), a dog in the Looney Tunes cartoon series
 * K9 Murphy, a mechanical dog in the Japanese television series Tokusou Sentai Dekaranger

Computing

 * AMD K9, a microprocessor
 * K-9 Mail, a mail client for the Android operating system
 * K9 Web Protection, a web content control software
 * K9Copy, a DVD backup and authoring program for Unix-like operating systems

Entertainment

 * K-9 (film), a 1989 American film
 * K-9 (film series), a film series consisting of four movies
 * K-9 (TV series) a British/Australian comedy/adventure series starring the same character as featured in Doctor Who
 * K-9 and Company earlier British pilot for a potential adventure series, also starring the character from Doctor Who.
 * K9, a television station serving Boise, Idaho, now known as KNIN-TV
 * Sonata in G for Keyboard and Violin, K. 9, a sonata by Wolfgang Amadeus Mozart

Military

 * HMAS K9, an Australian submarine
 * K-9 (missile), a Soviet short-range air-to-air missile
 * Kahr K9, a variant of the Kahr K series, a 9×19mm Parabellum semi-automatic pistol manufactured by the American company Kahr Arms
 * K9 Thunder, a 155 mm self-propelled artillery used by the Republic of Korea Armed Forces
 * Pusan East (K-9) Air Base, an abandoned air base in Busan, South Korea

Automotive

 * Kia K9, a full-size luxury sedan
 * K-9 (Kansas highway), a state highway in Kansas, US
 * BYD K9, a battery electric bus

Other

 * K9 glass, an inexpensive variety of crown glass
 * K9, a children's shoe size