Reynolds 531 (pronounced 'five-three-one') is a brand name, registered to Reynolds Technology of Birmingham in the United Kingdom, for a manganese–molybdenum, medium-carbon steel tubing that was used in many quality applications, including race car chassis, aircraft components and, most famously, bicycle frame tubing.

Reynolds also made up complete 'sets' of tubing for different cycling applications – for example 531c (Competition), 531st (Super Tourist), 531ATB (All Terrain Bike) and so-on.

The widespread use of TIG and MIG welding in cycle manufacture became a problem as 531 reacted poorly to the higher temperatures produced and 531 has been gradually phased out as a result.

[2] The most common like-for-like replacements for 531 are Reynolds 520 and 525 – a chrome-molybdenum tubing with very similar characteristics, but in addition to brazing, can also be welded.

Its mechanical properties and response to heat treatment are broadly similar to the AISI 4130 standard alloy steel, also used for bicycle frames, motorcycles, as well as aviation and motor-sport.