California has raised the legal age for buying tobacco products from 18 to 21, as part of anti-smoking legislation that also regulates e-cigarettes.
The law makes it illegal to sell tobacco products to people under 21, except to active military personnel.
Supporters of the law say it will deter adolescents from the harmful effects of nicotine addiction.
Under the measures signed on Wednesday, electronic cigarettes, like traditional ones, will be banned in public spaces.
Democratic Governor Jerry Brown signed a total of five bills to restrict tobacco use in various ways, including regulating e-cigarettes and expanding funds for anti-smoking programmes.
From 6 June, any company caught selling tobacco products for smoking, dipping, chewing and vaping (inhaling vapour through a personal inhaler, similar to an electronic cigarette) to under-21s will be fined up to $5,000 (£3,450).
Health officials have welcomed the legislation, saying it will stop adolescents from becoming addicted to nicotine.
“It is long past due for California to update our approach to tobacco,” said Steven Larson, president of the California Medical Association. “There has been an alarming rise in the use of e-cigarettes by teens, putting them at risk for lifelong addiction.”
The Institute of Medicine reports that 90% of daily smokers began using tobacco before turning 19.
It said increasing the smoking age to 21 would immediately deter 15% of people between 18 and 20 from taking up a lasting tobacco habit.
Veterans’ organisations and Republican lawmakers had objected to the bill, saying people old enough to die for their country were old enough to use tobacco.
The California proposal stalled for six months until lawmakers agreed to retain the 18-year-old legal tobacco age for military personnel and passed it in early March.
The tobacco industry has so far not commented, but it has previously threatened to try and overturn the bill. It would need to collect 366,000 valid signatures by early August to ask voters to reject the new laws.
In April, Hawaii became the first US state to raise the legal smoking age to 21.
Local authorities in the US including New York, Chicago and San Francisco have also made the change.