The Atonement star has posed for a photograph holding a banner carrying the slogan "Protect the Human", released to mark the 60th anniversary of the United Nations' Universal Declaration of Human Rights (Udhr).
The agreement, which was signed on 10 December 1948, set out for the first time in a single document the fundamental rights that should be enjoyed by all.
Knightley has also made a short film to mark the occasion, using the opportunity to voice her opinion on the matter.
She says, "I wanted to be part of this film for Amnesty to help raise awareness of the Udhr and to help them, in a small way, to campaign against the abuses of human rights that are still happening every day.
"It seems that 'human rights' has become a bit of a loaded term in this country, but if you look at the declaration that countries made 60 years ago, it just sets out a series of basic rules about how people should treat each other.
"The Udhr is something that everyone should be aware of and be proud of as a statement of our common humanity."