Prof Hawking, 71, who has motor neurone disease, said "We don't let animals suffer, so why humans?".
In an interview he said he believed those who helped loved ones die should escape prosecution if they were suffering from a terminal illness and in pain.
It is a criminal offence in the UK to encourage or assist someone to take their own life but guidance, issued by the Department of Public Prosecutions in 2010, makes it clear that friends or family members are unlikely to be prosecuted for assisting a loved one’s suicide.
However campaigners have called for greater clarity in the law.
Prof Hawking said: “I think those who have a terminal illness and are in great pain should have the right to chose to end their own life and those that help them should be free from prosecution."