SIN can refer to:

• 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

In religious contexts, a sin is an act that violates God's will.

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 Valve Software's Steam Platform on April 5, 2006, either as a standalone product, or bundled together with its sequel, SiN Episodes.