The Los Angeles Police Department said it responded at 4:30 p.m. local time to a building on campus.
Fire Department spokeswoman Margaret Stewart told The Los Angeles Times paramedics found Tjan dead at the scene.
LAPD Det. Meghan Aguilar told the L.A. Times that Tjan had multiple stab wounds.
Police said the suspect, a male student, is in custody.
"We want to make clear this was not a random act," Aguilar said. "This victim was targeted by the suspect."
The name of the suspect has not been released.
Chris Purington, project manager at Tjan's lab, told the AP that he never heard of anyone having a problem with the professor.
"People talk about scientists as very cold or robotic. Bosco is a guy that he could talk to anybody about anything," he said. "He couldn't move through a room without being sidetracked in all these conversations.
"He just had this energy about him. Kinetic might be the word," Purington said. "He had a huge impact on my life."