303 is a year of the Julian calendar.

303 may also refer to:

• 303 (number)
• 303 BC, a year of the pre-Julian Roman calendar
• Area code 303, a telephone code assigned to central Colorado
• Roland TB-303, a bass synthesizer
• .303 British, the .303" calibre rifle and machine-gun cartridge
• 303 (comics), comic book miniseries by Garth Ennis and Jacen Burrows
• 303 Josephina, a main-belt asteroid
• Prometheus (Stargate), also called BC-303 or X-303, fictional spacecraft in the Stargate SG-1 television show
• 3OH!3, an electropop band from Boulder, Colorado
• "303", also used in Australia and New Zealand to refer to the Lee–Enfield rifle
• "303", a song by Kula Shaker from the album K

A303 road, a British Highway that is the subject of the Kula Shaker song

• HTTP 303, a status code for "See other"
• SOP 303, a National Communications System standard operating procedure to turn off mobile communications
• 303 (group), a British girl group
• Pearson 303, an American sailboat design
• 303 Squadron (disambiguation), several military units