The Spinnaker Tower is a 170-metre (560 ft) landmark observation tower in Portsmouth, England. It is the centrepiece of the redevelopment of Portsmouth Harbour, which was supported by a National Lottery grant. The tower's design was chosen by Portsmouth residents from a selection of three different designs in a 1998 public poll. It has three viewing platforms one on top of the other at heights of 100 m, 105 m and 110 m.

The tower was designed by local firm HGP Architects and engineering consultants Scott Wilson and built by Mowlem with steelwork by Butterley Engineering. The Spinnaker Tower reflects Portsmouth's maritime history through its design and is named after a spinnaker, a type of sail that balloons outward. The tower was opened on 18 October 2005.

The tower is owned by Portsmouth City Council and is operationally managed by Continuum Leading Attractions, a cultural attractions group based in York. Continuum also runs five other visitor attractions across the country.

The Spinnaker Tower was repainted and rebranded as the "Emirates Spinnaker Tower" from July 2015 following a five-year commercial sponsorship deal with Dubai-based Emirates airline. The Emirates sponsorship deal expired in 2020 and the tower reverted to its original all-white paint scheme and name in April 2021. On 5 May 2023, it was announced that Macmillan Cancer Support would become the tower's new sponsor beginning in June.

At a height of 560 feet (170 m), the tower is one of the tallest accessible structures in the United Kingdom outside London. The tower is visible around Portsmouth, changing the horizon of the area. It can be seen from the Isle of Wight, the Manhood Peninsula, Highdown Gardens and Cissbury Ring in Worthing and Nine Barrow Down in the Purbeck Hills, Dorset.

The tower represents sails billowing in the wind, a design accomplished using two large, white, sweeping metal arcs, which give the tower its spinnaker sail design. The steelwork was fabricated by Butterley Engineering. At the top is a triple observation deck, providing a 360° view of the city of Portsmouth, the Langstone and Portsmouth harbours, and a viewing distance of 37 kilometres (23 mi). The highest of the three observation platforms, the Sky Deck, has only a wire mesh roof, so visitors are open to the elements. The windows extend above head height, so it is not possible to get a view unobstructed by glass. A glass floor is located on the first viewing deck at 100 metres above sea level. It has three viewing platforms one on top of the other at heights of 100 m, 105 m and 110 m. The tower has a design lifetime of 80 years.

Portsmouth's Millennium Tower, as it was originally intended to be named, was a project conceived in 1995 to design a monument to commemorate the Millennium celebrations in the year 2000. It received UK National Lottery funding and was intended to open in late 1999.

A choice of three different Millennium Tower designs were presented to the people of Portsmouth for a public poll in early 1998.