Researchers said those numbers are similar to the prevalence of lifetime suicidal thinking and attempts reported by adults - suggesting the teenage years are an especially vulnerable time.
"What adults say is, the highest risk time for first starting to think about suicide is in adolescence," said Matthew Nock, a psychologist who worked on the study at Harvard University in Cambridge, Massachusetts.
The results are based on in-person interviews of close to 6,500 teens in the U.S. and questionnaires filled out by their parents. Along with asking youth about their suicidal thinking, plans and attempts, interviewers also determined which teens fit the bill for a range of mental disorders.
Just over 12 percent of the youth had thought about suicide, and four percent each had made a suicide plan or attempted suicide.
Nock and his colleagues found that almost all teens who thought about or attempted suicide had a mental disorder, including depression, bipolar disorder, attention deficit hyperactivity disorder (ADHD) or problems with drug or alcohol abuse.
More than half of the youth were already in treatment when they reported suicidal behavior. Nock said that was both "encouraging" and "disturbing."
"We know that a lot of the kids who are at risk and thinking about suicide are getting (treatment)," he told Reuters Health. However, "We don't know how to stop them - we don't have any evidence-based treatments for suicidal behavior."
Who is at risk?
Amy Brausch, a psychologist who has studied adolescent self-harm and suicide at Western Kentucky University in Bowling Green, said the finding shouldn't be interpreted to mean mental health treatment doesn't work for teens.
"We don't know from this study if they even told their therapist they were having these thoughts, we don't know if it was a focus of the treatment," Brausch, who wasn't involved in the new research, told Reuters Health.
The findings were published this week in JAMA Psychiatry. But they still leave many questions unanswered.
Because most youth who think about suicide never go on to make an actual plan or attempt, doctors need to get better at figuring out which ones are most at risk of putting themselves in danger, according to Nock.
Once those youth are identified, researchers will also have to determine the best way to treat them, he said - since it's clear that a lot of current methods aren't preventing suicidal behavior.
According to the U.S. Centers for Disease Control and Prevention, suicide is the third leading cause of death for people between age 10 and 24, killing about 4,600 young people annually.
Although girls are more likely to attempt suicide - a pattern confirmed by Nock's study - boys have higher rates of death by suicide because they typically choose more deadly methods, such as guns.
For parents, Brausch advised keeping open communication with their adolescent and not being afraid to ask about things like depression. They should also pay attention to changes in mood or behavior, she added.
Nock agreed. "For parents, if they suspect their child is thinking about suicide… or talking about death, I would have that child evaluated," he said.