Livermore police wrote on Twitter at 11:34 a.m. that they were investigating a "suspicious occurrence" at the campus at 3000 Campus Hill Drive.
Shortly after noon, police wrote that the lockdown had been lifted and later said the Nerf gun was what prompted the response.
Police said the student had "no intent to cause harm or problems."
Las Positas College officials took to Twitter to announce that classes and activities on campus were restarting.
Click here to get the latest updates from the college from their website.