Supporters say the law will help protect teens from ultraviolet rays that could lead to skin cancer.
"Making sure families understand the risks associated with indoor tanning is a common sense measure that will improve public health," Gov. Nixon said. "Exposure to ultraviolet light from tanning beds puts children and teenagers at a much higher risk of developing skin cancer later in life, and this legislation will bring Missouri in line with the large and growing number of states taking steps to prevent cancer and protect kids."
Opponents argue the law puts restrictions on small businesses.
Salons that allow teens to tan without parental consent could face a $100 fine for a first time offense.
Six states ban tanning for teens.