A thermosetting composition including a thermosetting compound and hexagonal boron nitride D, wherein the thermosetting compound contains a cyanate compound A and/or a maleimide compound B, and a modified polyphenylene ether C having a substituent with a carbon-carbon unsaturated double bond at at least one terminal, and a content of the hexagonal boron nitride D is 0.1 parts by mass or more and 25 parts by mass or less based on 100 parts by mass of the thermosetting compound.