This paper proposes robust control synthesis for homogeneous leaderless multi-agent systems (MASs) subjected to parametric uncertainty and exogenous disturbance. We employ an approach of robust H2 hierarchical decentralized control. This approach utilizes a linear quadratic performance criterion which composes of local and global terms. Local term represents local control action of agents whereas the global term deals with cooperation among agents. We formulate design of the local and global feedback control laws as optimization over linear matrix inequality (LMI). Furthermore, the LMI formulation can incorporate a specified constraint on disturbance attenuation. Finally, a design procedure and numerical examples are provided to illustrate the effectiveness of the proposed robust H2 design approach.