A series of graphene nanoplatelet-modified acrylonitrile-butadiene rubber-based underwater acoustic absorbing materials were prepared. The dynamic mechanical properties, underwater sound absorption properties, differential scanning calorimetry, vulcanization property, and mechanical properties of graphene nanoplatelets/acrylonitrile-butadiene rubber nanocomposites were studied theoretically and experimentally. The results indicated that graphene nanoplatelet-modifiedacrylonitrile-butadiene rubber-based underwater acoustic absorbing materials exhibited excellent damping and underwater sound absorption properties. The storage modulus ([Formula: see text]) and loss modulus ([Formula: see text]) of graphene nanoplatelets/acrylonitrile-butadiene rubber nanocomposites were increased significantly with increasing graphene nanoplatelets content. At a graphene nanoplatelets content of 25 phr, the [Formula: see text] and [Formula: see text] at 15°C improved by 1201 and 603%, respectively. The obvious improvement in [Formula: see text] and [Formula: see text] were mainly attributed to the extremely high interfacial contact area between graphene nanoplatelets and acrylonitrile-butadiene rubber chains and the ultrahigh mechanical properties of graphene nanoplatelets. The underwater sound absorption coefficient ([Formula: see text]) was increased obviously as the graphene nanoplatelets were incorporated. The optimal [Formula: see text] of the nanocomposites was achieved as the graphene nanoplatelets content was 10 phr, and the average value of [Formula: see text] was improved from 0.35 to 0.73—an increase of nearly onefold. The notable improvement in [Formula: see text] was due to the marked increase in damping properties and thermal conductivity of graphene nanoplatelets/acrylonitrile-butadiene rubber nanocomposites. The merits of graphene nanoplatelet-modified underwater acoustic absorbing materials were higher damping, better underwater sound absorption, and better mechanical properties with unaffected density in comparison to other inorganic and rigid fillers or porous fillers.