Madonna brought her husband and daughter along
She was accompanied to Tuesday night's event in London by her five-year-old daughter Lourdes and husband Guy Ritchie.
Testino is best known for his work in fashion photography but has become much sought after by showbusiness stars.
His subjects have included model Kate Moss, pop stars Madonna and Robbie Williams and actresses Liz Hurley and Gwyneth Paltrow.
Many of the celebrities and models to have been captured by Testino feature in the exhibition - and were among the launch guests.
The exhibition displays more than 120 colour and black and white portraits selected by Testino himself.
This is Testino's first show at a major London institution. Many of the pictures have not been seen in public before.
Special sections are devoted to Madonna, Moss and Paltrow. One of his pictures of Madonna features Lourdes.
Other photographs are of Catherine Zeta Jones, model Gisele and designers Tom Ford, John Galliano, Stella McCartney and Alexander McQueen.
Testino was Princess Diana's favourite fashion photographer and some images of the princess, taken shortly before her death in 1997, are also on display.
Born in Lima, Peru, Testino did not pursue a career in photography until he moved to London to begin formal training in the early 1980s.
He originally studied Economics, Law and International Relations in Peru and California.
Though permanently based in London, his work for various international fashion magazines including Vogue and Vanity Fair requires him to travel to France, Italy, Britain and the US.
The exhibition will run from 1 February until 4 June 2002.