Dreaming about a costume signifies various facets of your waking life and personality. What costume your wear is often associated with a given position, such as Halloween attire or school uniform. If you wear a mask, it may be associated with espionage or other operations.
Also, dreaming about costumes indicate that you’re exploring who you are or what society expects of you. You may be dressing up in your waking life to demonstrate who you believe yourself to be, or who others expect of you.
On the negative side, dreaming about wearing a costume could represent one not being aware of themselves and the way they present themselves to others. In other words, it symbolizes that one is a fraud and seeks acceptance but this will never happen if they don’t completely accept themselves without seeking approval.