Two new 3-keto-4-methylene steroids, theonellasterone (3) and conicasterone (4), and a Diels-Alder type dimeric steroid, bistheonellasterone (5), were isolated together with two known 4-methylene sterols, theonellasterol (1) and conicasterol (2), from the Okinawan marine sponge Theonella swinhoei. The structures of these steroids have been elucidated on the basis of chemical and physicochemical evidence. Bistheonellasterone (5) is considered to be biosynthesized through a Diels-Alder cycloaddition of theonellasterone (3) and its Δ⁴-isomer. Very interestingly, theonellasterone (3) and conicasterone (4) were seen under an optical microscope as crystals deposited in the tissue of fresh marine sponge.