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 (
) and loss modulus (
) of graphene nanoplatelets/acrylonitrile-butadiene rubber nanocomposites were increased significantly with increasing graphene nanoplatelets content. At a graphene nanoplatelets content of 25 phr, the
and
at 15°C improved by 1201 and 603%, respectively. The obvious improvement in
and
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 (
) was increased obviously as the graphene nanoplatelets were incorporated. The optimal
of the nanocomposites was achieved as the graphene nanoplatelets content was 10 phr, and the average value of
was improved from 0.35 to 0.73—an increase of nearly onefold. The notable improvement in
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.