Abstract: An object is to provide an electrolyte having stretchability and flexibility and capable of preventing a decrease in durability of the electrolyte, a secondary cell, and a composite material. The object can be implemented with an electrolyte containing: a polymer obtained by polymerizing a monomer represented by the following Formula (1) (In Formula (1), R1 and R2 each independently represent H or a linear, branched, or cyclic alkyl group having 1 to 20 carbon atoms. X1 and X2 each independently represent O or NH. When X2 is O, n represents an integer of 0 to 30 on average, and when X2 is NH, n represents an integer of 1 to 30 on average.); a glyme represented by the following Formula (2) (In Formula (2), R3 and R4 each independently represent an alkyl group having 1 to 4 carbon atoms, and m represents an integer of 1 to 4.); and at least one salt selected from the group consisting of a lithium salt, a sodium salt, a magnesium salt, a potassium salt, and a calcium salt.