SIN can refer to:

• Sex Industry Network (SIN)
• the trigonometric sine function
• Singapore, FIFA trigramme and International Olympic Committee country code
• Singapore Changi Airport, IATA airport code
• Social Insurance Number, a number which may be issued to a person in Canada in furtherance of various government programs
• Sistema de Inteligencia Nacional, the Argentine intelligence services
• Spanish International Network, a Spanish-language American television network that predated Univision

SiN is a first-person shooter video game based on a modified version of the Quake II engine developed by Ritual Entertainment and published by Activision in late 1998. The game was later released over Steam platform on April 5, 2006, either as a standalone product, or bundled together with its sequel, SiN Episodes. It was also released, along with its expansion pack on GOG.com on January 30, 2014, DRM-free and fixed for modern hardware.

In a religious context, sin is the act of violating God's will. Sin can also be viewed as anything that violates the ideal relationship between an individual and God; or as any diversion from the perceived ideal order for human living. To sin has been defined as "to miss the mark".