The grenade still had the pin in it, but it was later determined to be "inert and safe," school district spokeswoman Julie Thannum said. A bomb squad was called in to remove it from a classroom.
CBS station KTVT in Dallas was told that the child got the grenade as a souvenir from someone who had recently returned from Iraq.
"The boy wasn't mad at anyone," Cpl. Mike Bedrich of the Southlake Department of Public Safety told the Fort Worth Star-Telegram newspaper. "He just thought it would be cool to bring it to school."
School staff and 530 students were allowed to return to Old Union Elementary School a little over an hour later.
Thannum declined to say if any action would be taken against the student.