Federal law enforcement sources say someone emailed a bomb threat and gave four locations that the alleged devices were placed: the Science Center, Thayer, Sever and Emerson Halls.
All four locations were evacuated as a precaution. Harvard University police and Cambridge police responded to the scene and searched the campus.
Final exams had just begun at the Ivy League school, but some exams were canceled due to the threat, according to the school's paper, The Harvard Crimson.
"As of the writing of this message the report remains unconfirmed and the HUPD has no reason to believe there is a threat to any other site on campus," the school posted on its website.
At 2:44 p.m. ET, all four locations were cleared and reopened to students.
Last month, another Ivy League school, Yale University in Connecticut, was locked down for nearly six hours while authorities investigated a phone call saying an armed man was heading to shoot it up, a warning they later said was likely a hoax.
In February, someone called in a hoax about a gunman on the campus of the Massachusetts Institute of Technology, another elite school about two miles from Harvard. The university said the gunman was a staff member looking for revenge after the suicide of an Internet activist accused of illegally using MIT computers.
The Associated Press contributed to this report.