Provided are: a syringe cap that allows an insertion needle or the like to be sufficiently sterilized and the amount of a filled agent to be maintained； a syringe assembly； and a prefilled syringe. The syringe cap (10) is detachably attached to an outer tube in such a manner to seal a needle pore (26) of an insertion needle (18) held by a tube tip section (16) of the outer tube (14). This syringe cap (10) comprises: a first member (30) into which the insertion needle (18) is inserted, the first member comprising an elastic body having a Young's modulus of 1 to 10 MPa； and a second member (32) comprising an elastic hollow body having a Young's modulus of 1 to 10 MPa and forming a closed space between the first member (30) and the outer tube (14). The first member (30) is formed from a material having a smaller moisture-vapor transmission rate compared to the second member (32).
