The Audi RS6 is the highest performing version of the Audi A6, a mid-size sports car-focussed executive car produced by German automaker Audi. It is only available in quattro all-wheel drive. Production of the first-generation RS6 began in June 2002 and ended in October 2004. The second-generation was introduced at the 2007 Frankfurt Motor Show.

Its main rivals are the Mercedes-Benz E-Class AMG and BMW M5. The C5 RS6 (2004) was the third model to come out of Audi's quattro GmbH; first was Porsche/Quattro RS2 Avant, second the B5 RS4 (2001); fourth the A4 DTM edition saloon and the current is the B7 RS4 (2006). It was the first RS model to reach the United States and Canada aside from many owners converting the B5 S4 Saloon version into an RS4 spec 4 door replica. The "RS" stands for "Renn Sport", and is Audi's ultimate trim level, positioned above the "S" ("Sport") line of Audi's regular line-up. The first RS6 was the top-of-the-line of C5 (2nd generation, 1997-2004) A6 and was available as both a sedan and a wagon (called Avant by Audi). Derived from the C5 S6, the RS6 also shares aluminium structure from the firewall and forward with the C5 V8 A6s (A6 4.2 V8 quattro, S6 sedan/Avant).