They asked them how many times they checked their phones and other digital devices for various reasons, and then monitored them for symptoms of ADHD, including impulsivity, hyperactivity, and inattention.
Social media was the most common reason for checking a digital device, with text messaging, music streaming and video chatting other common uses.
After two years, those who had checked their phones the most often were twice as likely as those who checked the least often to show signs of ADHD.
Writing in the JAMA medical journal, the scientists said: "Modern media devices immediately notify users when new text messages, social media postings, or video game play invitations arrive.
"Exposure to such notifications may draw attention away from focal tasks.
"Frequent distractions could disrupt normative development of sustained attention and organisation skills."
They believe constant access to instant entertainment also has an impact.
"High-frequency modern digital media users may become accustomed to rapid feedback, which could disrupt development of impulse control and patience," they wrote.
Researcher Professor Adam Leventhal said all previous research had focused on the link between ADHD and televisions.
"What's new is that previous studies on this topic were done many years ago, when social media, mobile phones, tablets and mobile apps didn't exist," he said.
"New, mobile technologies can provide fast, high-intensity stimulation accessible all day, which has increased digital media exposure far beyond what's been studied before.
"We can say with confidence that teens who were exposed to higher levels of digital media were significantly more likely to develop ADHD symptoms in the future."
He said the findings help fill a gap in understanding how new, mobile media devices and seemingly limitless content pose a mental health risk to children. And the findings serve as a warning as digital media becomes more prevalent, faster and stimulating.
"This study raises concern whether the proliferation of high-performance digital media technologies may be putting a new generation of youth at risk for ADHD," Professor Leventhal said.
British scientists last night welcomed the findings.
But Professor Andy Przybylski of the Oxford Internet Institute said: "The study relies on survey responses provided by the student in question.
"It is not clear if teachers or parents would rate the children similarly or if the self-reported measure of digital screen use is correlated with either actual behaviour or higher quality survey items."
Dr Jessica Agnew-Blais of King's College London added: "It is worth noting that over 80 per cent of students reported high frequency use of digital media, and the vast majority of these students do not have elevated ADHD symptoms."
