Since 2016, California has allowed children under 18 to receive taxpayer-backed health care despite immigration status.
The idea of giving health benefits to undocumented immigrants is supported by most of the Democratic candidates running for president, and California's move comes as the Trump administration continues to ramp up its hard-line crackdown on illegal immigration. On Tuesday, Newsom said the state law draws a sharp contrast with Trump's immigration policies.
"If you believe in universal health care, you believe in universal health care," Newsom said. "We are the most un-Trump state in America when it comes to health policy."
In California, extending health benefits to undocumented immigrants is widely popular. A March survey conducted by the nonpartisan Public Policy Institute of California found that almost two-thirds of state residents support providing coverage to young adults who are not legally authorized to live in the country.
California, the group notes, has more immigrants than any other state. And an estimated 14% of them do not have legal status.
A national survey suggests that many Americans across the country are far less accepting of the notion of giving health coverage to those who came into the U.S. illegally. A CNN poll conducted after the Democratic debates last month found that 59% of respondents do not think government-backed health coverage should be provided to undocumented immigrants.
In most states, people living in the country illegally are not eligible for federal health insurance programs like Medicaid and Medicare, except in some cases, like medical emergencies and pregnancies, according to the National Conference of State Legislatures.
Republican lawmakers in California criticized the law, arguing that the state should be spending health care dollars on those living in the state legally.
"We are going to be a magnet that is going to further attract people to a state of California that's willing to write a blank check to anyone that wants to come here," said state Sen. Jeff Stone, R-Temecula, at a May legislative hearing. "We are doing a disservice to citizens who legally call California their home."