After the controversial bill on euthanasia came into effect, California becomes the fifth US state to allow physician-assisted suicide, The bill now lets terminally-ill patients seek doctor's help to end their lives.
Governor Jerry Brown said he consulted members of the Catholic Church, which is opposed to measure, as well as physicians, before making the decision.
"In the end, I was left to reflect on what I would want in the face of my own death. I do not know what I would do if I were dying in prolonged and excruciating pain. I am certain, however, that it would be a comfort to be able to consider the options afforded by this bill. And I wouldn't deny that right to others," said the 77-year-old governor.
The bill makes California the fifth US state to allow assisted suicide after Montana, Oregon, Washington and Vermont.
Euthanasia has long been a controversial issue in the United States. The topic was brought to the forefront in California by the case of Brittany Maynard, a 29-year-old with a brain tumor who moved from San Francisco to Oregon and took her own life last November.