The letter comes days after Klein - who led Northwestern's prestigious Medill Justice Project -- took a leave of absence, pending an investigation.
A group of former students have alleged harassment, misconduct and bullying by Klein.
The faculty letter said they are committed to doing their part to ensure that this doesn't happen again. The letter said: "it may not be visible to you yet, but the allegations you made shook many of us to the core. In hallways, in classrooms, in meetings among faculty, we have begun a period of profound reflection of the issues you've raised."
The open letter also accuses the university of turning a blind eye to the professor's actions. Northwestern officials said they have begun investigation.
Klein, who has worked at Northwestern since 2008, denied the allegations, who said they stem from a disgruntled former employee.
Nevertheless many students say they believe the allegations and are glad other professors are coming forward.