California has became the first US state to require that public school textbooks include the accomplishments of gay, lesbian and transgender Americans as governor Jerry Brown signed the mandate into law.
"History should be honest," Mr Brown said in a statement released by his office.
The measure won final passage from the state legislature earlier this month when it passed on a 49-25 party-line vote, with Democrats in favour and Republicans opposed.
"This bill revises existing laws that prohibit discrimination in education and ensures that the important contributions of Americans from all backgrounds and walks of life are included in our history books," Mr Brown said. "It represents an important step forward for our state.
The law also requires that public schools teach the contributions of Pacific Islanders and the disabled.
California already mandates that schools include historical accomplishments by Native Americans, African Americans, Mexican Americans, Asian Americans and European Americans.
Republicans who opposed the legislation argued that it would write an agenda into school textbooks.
Randy Thomasson, president of the conservative SaveCalifornia.com, said Mr Brown had "trampled the parental rights of the broad majority of California mothers and fathers who don't want their children to be sexually brainwashed".
"The only way parents can opt-out their kids from this immoral indoctrination is to opt them out of the entire public school system, which is no longer for morally-sensitive parents and their children," Mr Thomasson said.
It could still be several years before California students start reading about gay accomplishments in their textbooks.
The state's department of education has said that, because of budget problems, new textbooks will probably not be adopted until 2015.