HMNB Portsmouth

His Majesty's Naval Base, Portsmouth (HMNB Portsmouth) is one of three operating bases in the United Kingdom for the Royal Navy (the others being HMNB Clyde and HMNB Devonport). Portsmouth Naval Base is part of the city of Portsmouth; it is located on the eastern shore of Portsmouth Harbour, north of the Solent and the Isle of Wight. For centuries it was officially known as HM Dockyard, Portsmouth: as a Royal Navy Dockyard, Portsmouth functioned primarily as a state-owned facility for building, repairing and maintaining warships; for a time it was the largest industrial site in the world.

From the 1970s, the term 'Naval Base' began to be used for Portsmouth (and other Royal Dockyards), acknowledging a greater focus on personnel and support elements alongside the traditional industrial emphases. In 1984 Portsmouth's Royal Dockyard function was significantly downsized and downgraded, and was formally renamed the 'Fleet Maintenance and Repair Organisation' (FMRO). The FMRO was privatised in 1998; in 2002, shipbuilding (which had not taken place on site since the late 1960s) resumed in the form of block construction, but this again ceased in 2014.

Today, Portsmouth is the home base for two-thirds of the Royal Navy surface fleet, including the two aircraft carriers, HMS Queen Elizabeth and HMS Prince of Wales. Naval logistics, accommodation and messing are provided on site, with personnel support functions (e.g. medical and dental; education; pastoral and welfare) provided by Defence Equipment and Support. Other functions and departments, e.g. Navy Command Headquarters support staff, are also accommodated within the Naval Base. The base is additionally home to a number of commercial shore activities, including the ship repair and maintenance facility operated by BAE Systems Maritime Services.

The base is the oldest in the Royal Navy, and it has been an important part of the Senior Service's history and the defence of the British Isles for centuries. It is home to one of the oldest surviving drydocks in the world. The former Block Mills are of international significance, having been the first factory in the world to employ steam-powered machine tools for mass production. The Royal Naval Museum has been on the site since 1911. In 1985 a partnership between the Ministry of Defence and Portsmouth City Council created the Portsmouth Naval Base Property Trust to manage part of the historic south-west corner of the Naval Base, under a 99-year lease, as an heritage area, the Portsmouth Historic Dockyard. It allows members of the public to visit important maritime attractions such as Mary Rose, HMS Victory, HMS Warrior and the National Museum of the Royal Navy.

The Naval Base Commander (NBC) since September 2024 is Commodore Marcel Rosenberg. The harbour is under the control of the King's Harbour Master (KHM), who is the regulatory authority of the Dockyard Port of Portsmouth, an area of approximately 50 square miles (130 km²) that encompasses Portsmouth Harbour and the Eastern Solent. KHM Harbour Control is based in the Semaphore Tower building. Shipping movements are handled by a team of admiralty pilots headed by the Chief Admiralty Pilot.

In 1836 the Commander-in-Chief, Portsmouth was given accommodation within the Dockyard (in Admiralty House) and in 1889 he was given HMS Victory to be his ceremonial flagship. These privileges were inherited by the Commander-in-Chief Naval Home Command (whose post was combined with that of Second Sea Lord in 1994), who continued to fly his flag from HMS Victory until 2012. That year the post of Commander-in-Chief reverted to the First Sea Lord, and with it the use of Victory as flagship. The Second Sea Lord is now based at the Henry Leach Building on Whale Island, which is also the headquarters of the Fleet Commander.

Some of the following ships (e.g. the patrol vessels) are not based in Portsmouth, but form part of the Portsmouth Flotilla.

In changes to base porting arrangements announced in November 2017, HM Ships Westminster, Richmond, Kent and St Albans were to move to the HMNB Devonport by 2023; HM Ship HMS Argyll moved in the opposite direction. HMS Monmouth and HMS Montrose were also to move to Portsmouth. However, Monmouth retired in 2021, Montrose decommissioned in 2023 and Argyll and Westminster followed in 2024. Richmond became a Devonport ship on completion of her refit. St Albans moved to Devonport in July 2019 in preparation for her major refit. In 2025, it was confirmed that HMS Iron Duke would relocate from Portsmouth to the Devonport naval base. Once HMS Lancaster completed her Persian Gulf deployment in 2025, it was expected that she would decommission thus ending the basing of Type 23 frigates at Portsmouth.