.577 may refer to:

• .577 Black Powder Express, a black powder, centerfire round
• .577 Nitro Express, a rimmed metallic cartridge
• .577 Tyrannosaur, a type of cartridge developed by A-Square in 1993 for big game hunting in Africa
• .577/450 Martini–Henry, a black powder, centerfire round used by the British and British Empire militaries
• .577 Snider, an obsolete cartridge produced around 1867 and replaced in service by the .577/450 Martini–Henry cartridge
• The first three digits in the decimal expansion of Euler's constant, .5772156649...