Swimming at the 2003 World Aquatics Championships – Women's 4 × 100 metre medley relay

The Women's 4x100m Medley Relay event at the 10th FINA World Aquatics Championships swam 26 July 2003 in Barcelona, Spain. Preliminary heats swam in the morning session, with the top-8 finishers advancing to swim again in the Final that evening.

At the start of the event, the World (WR) and Championship (CR) records were:
 * WR: 3:58.30 swum by the USA on September 23, 2000 in Sydney, Australia.
 * CR: 4:01.50 swum by Australia on July 29, 2001 in Fukuoka, Japan.