Is it legal in the US for an employer to provide salary history to a potential employer who requests it while doing a reference check?

I don’t know the laws of every state, and it is certainly possible that some state restricts this, but the general answer is YES.

This is one of those areas of law where lay people often confuse policy with law. An employer has the right to say almost anything he or she wants about an employee, so long as the statement is truthful. There are some minor exceptions, but these exceptions almost always take the form of private facts divulged to the employer as a result of some official business.

For example, it would be illegal for an employer to divulge medical facts obtained during a workman’s comp investigation, or was divulged by the employee in order to receive an accommodation.

Wages do not fit in this category. Your employer knows how much you get paid because he or she is the one who decided to pay you that. Thus, they have the right to divulge it to anyone.

The misconception about what an employer can or cannot divulge comes from policy, and not law. Most employers have a policy that they will not divulge anything other than to confirm that the person worked at the company and confirm dates. They have this policy not because the law mandates it, but because there is no point in taking the risk of incurring litigation costs for an employee you already let go.

I worked in a large corporation's HR Dept. They would only give out employment dates of service to companies doing pre-employment reference checks - nothing more.

Salary history information is really none of anyone's business unless you personally want to reveal it on a job application though.

I believe the only legal question allowed is "would you rehire this employee again" and nothing else.

Yes it is usually current salary not the entire history.

Actually, it depends on the state. I think here in CA there are stricter safeguards in place - if a potential employer asks about salary, all they usually are able to say is "Joe X worked from [date] to [date]" and not even any salary or performance information.

The employer is taking a risk to divulge any information about the employee without the employee's express consent. Anyone can file a lawsuit against anyone else, including suing your former employer for giving out information about you.