Is however an adverb?

Yes, however can be classified as an adverb when used as follows:

Use Example
As a conjunctive adverb to mean “but” It is currently designated as agricultural land. However, this could change next year when new legislation is introduced.
To introduce a dependent clause to mean “no matter how” It needs to be finished today however long it takes.
Like an interrogative adverb to express surprise in a question However could you possibly think that?

You may need to include a comma before or after “however” depending on how and where it is used in a sentence.