RFA Wave Ruler is a Wave-class fast fleet tanker of the Royal Fleet Auxiliary (RFA) of the United Kingdom tasked with providing fuel, food, fresh water, ammunition and other supplies to Royal Navy vessels around the world.

Wave Ruler was built by Kvaerner Govan (after 1999, BAE Systems Marine) and launched in 2001. She was accepted into service in 2003 and is the second ship to bear this name in RFA service. Wave Ruler and her sister Wave Knight, were designed to replace Olna and Olwen, two 36,000-ton Ol-class fast fleet tankers which were built at Swan Hunter and Hawthorn Leslie in the 1960s.

Wave Ruler was the last tanker commissioned into the RFA until RFA Tidespring became operational in late 2017.

Wave Ruler has a standard crew of 80 Royal Fleet Auxiliary personnel with provision for a further 22 Royal Navy personnel to conduct helicopter and weapons systems operations. She carries a full medical team and sick bay and is capable of distributing 2,000 emergency relief packages in times of crisis.

The ship has the capability to supply fuel and other liquid cargo to vessels using replenishment rigs on port and starboard beams and through a Hudson reel-type stern rig. When providing support for amphibious operations, Wave Ruler is also able to deliver fuel to dracones positioned alongside. In addition to fuel, the ship carries ammunition and other stores which can be transferred while underway. She can operate a Merlin HM1 helicopter, or other helicopters of similar size, from a hangar and flight deck at the stern.

September 2004 saw Wave Ruler deployed to the Caribbean to provide hurricane relief operations.

In 2006 the ship carried out three major cocaine seizures at sea. In September it recovered £64m of cocaine from an estimated cargo of £500m, after the crew of the fishing boat carrying the drugs set it on fire. On 2 November the ship and its accompanying Royal Marines captured 3 tonnes (3.0 long tons; 3.3 short tons) of cocaine worth £300m. On 29 November it seized a further 2.9 tonnes (2.9 long tons; 3.2 short tons), again from a fishing boat. All the raids took place in the Caribbean.

On 31 August 2008 Wave Ruler was dispatched with HMS Iron Duke to assist relief efforts in the Caribbean for the Atlantic Hurricane Gustav. The vessels distributed food, water and first aid supplies to victims of the disaster as well as providing support restoring local infrastructure. On 3 October 2008 the ship docked in Havana, Cuba. This was only the second time since the country's revolution 50 years earlier that a Royal Navy ship had visited the country. The five-day stay was part of an ongoing anti-drugs operation in the Caribbean, which saw the ship spend much of 2005–2008 in the region, confiscating over 13 tonnes (13 long tons; 14 short tons) of cocaine in total. On 8 November 2008 the ship was sent to the Cayman Islands to provide humanitarian relief assistance in the wake of Hurricane Paloma.