Can I use a copyrighted song in a medley arrangement?

Generally, under U.S. copyright law, copyright owners have the exclusive right to create derivative works, which is new work that uses all or a part of a pre-existing work, and can include new works that combine two or more preexisting works (like a medley or mashup). Before using a work created by another party, you should contact the owner and request a license. You may also want to consult with an attorney who is knowledgeable with U.S. copyright law.