I'm not aware of any law in Massachusetts that prohibits an employer from doing this generally.  However, there are specific grounds for reducing salaries that are impermissible in Massachusetts.  For example, an employer may not reduce an employee's salary for a discriminatory reason (one that violates federal or state anti-discrimination laws).  Also, if the employer reduced your salary in retaliation against you for asserting some legal right, you might have legal recourse.  Also, you may be covered by an employment contract or collective bargaining agreement which prohibits such salary reductions.  These are just some examples.   A key question is why the employer decided to reduce your salary.  If the employer did not also reduce the salaries of "similarly situated" employees, you might have a cause of action.  There are a lot of variables which you would need to sort out -- a qualified attorney could help with that.