The following is provided as a radical-curable seal member for a fuel cell that has excellent productivity, can be made into a thin film, and has excellent seal ability within a wide temperature range. A radical-curable seal member 4 for a fuel cell having a glass transition temperature of -40°C or lower made from a crosslinked product of a radical-curable composition containing 20-90 weight parts of component (B), 1-10 weight parts of component (C), and 0.01-10 weight parts of component (D) per 100 weight parts of component (A), the weight ratio [(B)/(C)] of component (B) to component (C) being 4-24 and the content [(C)/{(A) + (B) + (C)}] of component (C) relative to a total of 100 wt% of component (A), component (B), and component (C) being 1-7 wt %. (A) (Meth)acrylic polymer having a (meth)acryloyl group at a molecular chain end. (B) Monofunctional (meth)acrylic monomer. (C) Polyfunctional (meth)acrylic monomer. (D) Radical polymerization initiator.