The hacker claimed to be a follower of hacker-activist group Anonymous, which crashed the websites of the Justice Department and major movie studios and recording labels last week.
The hacker at one point said the password to Grassley's account was "icecream" and then later said it was "chuck 123."
At about 2:30 p.m., Grassley's office appeared to regain control of the account, and deleted all of the hacked tweets.
Grassley's office did not immediately have a comment.
—Updated at 3:01 p.m.