In general, an employer can decrease your rate of hourly pay if you are an at-will employee.  However, in many states, the employer is required to first give you notice of the pay decrease.  Once you are notified, the employer can pay you at the lower rate.  In some states, this notice must be in writing.

Your employer cannot typically reduce your hourly wage for time you have already worked.  

Under no circumstances can your employer reduce your wages so that they are below either the state or federal minimum wage.

If My Wages Have Been Reduced Unlawfully, What Can I Do?

The legal action available to you depends on what state you are in.  In some states, a state agency is in charge of overseeing employment issues, and a dispute over reduced wages should be brought to their attention.  In other states, there is no state agency in charge of reductions in pay, so a private action against your employer is the only way to recover lost wages.  

In either scenario, you should contact an attorney familiar with employment law issues.  An employment attorney can help you determine whether to file a claim with a state agency or to bring a private lawsuit, and can help you to argue your case.