Rubber composition, crosslinked body, and tire

A rubber composition, comprising:

• (A) a polymer which has a carbon-carbon unsaturated bond, and in which a value a represented by the following equation (i) is 0.70 or more and 0.99 or less when p, q, r, and s are defined as constituent molar proportions, in the polymer, of a structural unit represented by the following formula (1), a structural unit represented by the following formula (2), a structural unit represented by the following formula (3), and a structural unit represented by the following formula (4), respectively;
• and
• (B) a thermoplastic resin,
• wherein the polymer (A) is contained in an amount of 60% to 95% by mass and the resin (B) is contained in an amount of 5% to 40% by mass relative to a total amount of the polymer (A) and the resin (B), the resin (B) is a styrene-based resin and the styrene-based resin is a block polymer having a conjugated diene-based polymer block and a polystyrene-based block,
• α=(p+(0.5×r))/(p+q+(0.5×r)+s) (i)