This paper addresses how to extend the concept of small-signal stability region (SSSR) to that of robust small-signal stability region (RSSSR), within which the system can remain stable even under perturbations caused by uncertain and volatile nodal injections, such as renewable generation. We first employ the structured perturbation theory to formulate the perturbations of nodal injections in state space. Both the intensity and the locations of perturbations can be taken into account. Then, we leverage the stability radius theory and structured singular value theory to define the RSSSR in parameter subspace, enabling a systematic analysis of small-signal stability of power systems under perturbations in a region-wise manner. The hyperplane-approximation method can be employed to construct a linear closed-form approximation of RSSSR boundaries. Case studies on the modified two-area system and New England 39-node system illustrate the new concept of RSSSR and its potential applications.