Lawyers have been preparing for over a year to challenge "illegal federal policy" that may be driven by Trump's government.
"The freedoms we value in California are under attack – and we won't just stand by and watch", said Newsom in a statement.
Newsom is urging California's lawmakers to prepare to combat "illegal" federal policy by, among other things, providing more funding to the Attorney General's office.
"California has faced this challenge before and we know how to act. We are ready to fight in the courts and we will do everything required to ensure that Californians have the support and resources they need."
During Trump's first term, California initiated dozens of lawsuits against the federal administration, ranging from issues on car emissions to building border barriers against Mexico, often together with other states, particularly the liberal west coast states of Oregon and Washington.
