An opportunistic storage service, or system, identifies currently unused storage capacity on a plurality of physical storage components of computing devices dispersed throughout a provider network. In some embodiments, the currently unused storage capacity is provisioned as primary storage, but is not currently being used to store primary storage data. The opportunistic storage service advertises at least a portion of the currently unused storage capacity as opportunistic storage capacity and provisions the opportunistic storage capacity subject to revocation if additional storage capacity of the physical storage components is needed to store primary storage data to fulfill a primary storage commitment.