The five-time Olympian won individual bronze at the 2004 Games - the first British female archer to earn a medal since 1908.
She has also medalled at World and European Championships, the Commonwealth Games and World Cup events during her long career.
She will become only the third Briton to compete in six summer Olympics.
Williamson quit her job as a primary school teacher to concentrate on the 2008 Olympics but suffered her worst performance when she failed to make the last 16 in Beijing.
Did you know?
She raised her profile by posing semi-nude in 1996 for photos that were displayed at the National Portrait Gallery in London. A strategically-placed bow and arrow preserved her modesty.