California's governor has signed into law a bill that allows euthanasia for terminally ill patients, the fifth US state to do so.

California has become the fifth US state to allow terminally ill patients to seek a doctor's help to end their lives after the governor signed a controversial bill.

Governor Jerry Brown, in a statement on Monday, said he consulted members of the Catholic Church, which is opposed to the 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," said the 77-year-old governor, who as a young man studied to enter the priesthood.

"I do not know what I would do if I were dying in prolonged and excruciating pain," Brown said.

"I am certain, however, that it would be a comfort to be able to consider the options afforded by this bill," he added.

"And I wouldn't deny that right to others."

The bill makes California the fifth US state to allow assisted suicide after Montana, Oregon, Washington and Vermont.

A New Mexico judge in 2014 approved assisted suicide, but his decision was later struck down by an appeals court.

The topic of euthanasia was brought to the forefront in California by the case of Brittany Maynard, a 29-year-old with a brain tumour.

In January 2014, Maynard was given six months to live and told her death would be painful because of the aggressive nature of her cancer.

She had been trying for a first child with her husband Dan Diaz at the time, but gave up because of her disease.

Maynard and her husband, who had just married when she began having severe headaches, moved from their home in California to Oregon, one of a handful of US states with a "right-to-die" law.

She ended her own life in November with medication prescribed by a doctor, surrounded by her family in the bedroom she shared with her husband.

The law is a controversial end to a debate that has roiled the nation's largest state by population for more than two decades.

California voters in 1992 rejected a proposal to allow doctors to administer lethal injections, and state legislators rejected assisted-suicide bills in 2005, 2006 and 2007, according to the Los Angeles Times.

Many religious groups still oppose assisted suicide on moral grounds, and the state legislature debated the bill for nine months before passing it in September, with a vote of 44-35 in the Assembly and 23-14 in the Senate.

The new law will take effect 90 days after the legislature adjourns a special session on health care, likely early in 2016.