The AgustaWestland AW101 is a medium-lift helicopter in military and civil use. First flown in 1987, it was developed by a joint venture between Westland Helicopters in the United Kingdom and Agusta in Italy in response to national requirements for a modern naval utility helicopter. Several operators, including the armed forces of Britain, Denmark, and Portugal, use the name Merlin for their AW101 aircraft. It is manufactured at factories in Yeovil, England, and Vergiate, Italy. Licensed assembly work has also taken place in Japan and the United States.

Prior to 2007, the aircraft had been marketed under the designation EH101. The original designation was EHI 01, from the name given to the Anglo-Italian joint venture—European Helicopter Industries—but a transcription error changed this to EH101. In 2000, Westland Helicopters and Agusta merged to form AgustaWestland, leading to the type's current designation.

The AW101 entered into service in 1999 and has since replaced several older helicopter types, such as the Sikorsky SH-3 Sea King, performing roles such as medium-sized transport, anti-submarine warfare, search and rescue, and ship-based utility operations. The Royal Canadian Air Force (RCAF) operates the CH-149 Cormorant variant for air-sea rescue. Another variant, the Lockheed Martin VH-71 Kestrel, was produced to serve in the United States presidential transport fleet before the program was cancelled and the aircraft sold off to Canada for parts. Civil operators use the AW101 for passenger and VIP transportation. The type has been deployed to active combat theatres, such as in support of coalition forces during the Iraq War and the war in Afghanistan.

In 1977, the UK Ministry of Defence issued a requirement for an anti-submarine warfare (ASW) helicopter to replace the Royal Navy's Westland Sea Kings, which were becoming inadequate in the face of advances in Soviet submarine technology. Westland Helicopters put together a proposal, designated WG.34, for a three-engined helicopter of similar dimensions to the Sea King; the WG.34 was to feature more autonomy and a greater range than its predecessor. At the same time, the Italian Navy (Marina Militare) was also considering the replacement of its fleet of Sea Kings, which had been built by the Italian company Agusta; Westland and Agusta soon began talks regarding the joint development of a successor helicopter.

Agusta and Westland finalised an agreement to work on the project together, and formed a jointly owned new company, EH Industries Limited (EHI), to pursue the development and marketing of the new helicopter to potential operators. On 12 June 1981, the UK government confirmed its participation in the project, allocated an initial budget of £20 million to develop nine pre-series examples. A major agreement, which secured funding for the majority of the EH101's development program, was signed by both the British and Italian governments in 1984. At the 1985 Paris Air Show, Agusta showed a mock-up of a utility version of the new helicopter, leading to a more generalised design that could be customised to meet the needs of various civilian or military customers. The first prototype flew on 9 October 1987.

In 1987, Canada selected the EH101 to replace its Sea Kings in the anti-submarine warfare and search and rescue roles. The EH101's third engine and increased range compared favourably with rival aircraft, such as the Sikorsky Seahawk. The range and de-icing capability were also seen as vital for North Atlantic operations. The fledgling EH101, of which up to 50 were on order to replace the Canadian Armed Forces's (CAF) Sea Kings, found itself subject to a wider political battle between the country's Conservative and Liberal parties, the latter viewing the aircraft as too costly. Critics attacked the EH101 purchase as excessive and unnecessary after the Cold War's end in the early 1990s. Wide-ranging cost estimates were presented by both proponents and opponents of the EH101 procurement, with opponents backing life extensions of Sikorsky CH-124 Sea King and Boeing Vertol CH-113 Labrador helicopter fleets. The EH101 controversy was seen as a factor in the 1993 Canadian federal election. The order was cancelled by the new Liberal government in 1993 resulting in a $470 million cancellation fee.

Several pre-production aircraft were assembled during the late 1980s and early 1990s. The first pre-production aircraft had its first flight powered by General Electric CT7-2A engines on 9 October 1987. Flight tests were suspended for six months following the crash of the second pre-production aircraft on 21 January 1993. On 6 June 1993, the first EH101 took its maiden flight with the Rolls-Royce Turbomeca RTM322 turboshaft engine. Nine prototypes were built to explore military and civil applications, including a "heliliner" configuration. In February 1995, Britain formally placed its first order for a total of 22 EH101s; this was quickly followed by Italy's order for 16 EH101s in October 1995. The EH101 order was not without controversy, the RAF had declared its preference for an all-Chinook fleet; also, Boeing allegedly offered cheaper terms for the Chinook. RAF deliveries began in 1997; RN deliveries started the following year.

Following the merger of Westland and Agusta to form AgustaWestland in 2000, there was no need for the separate EHI entity; the EH101 was formally re-branded as the AW101 in 2007. By April 2009, more than 180 AW101s had been sold worldwide; the combined operational fleet had also accumulated a total of 170,000 flying hours.