A resource-adaptive supernet adjusts its subnets for inference to fit the dynamically available resources. In this paper, we propose Prioritized Subnet Sampling to train a resourceadaptive supernet, termed PSS-Net. We maintain multiple subnet pools, each of which stores the information of substantial subnets with similar resource consumption. Considering a resource constraint, subnets conditioned on this resource constraint are sampled from a pre-defined subnet structure space and high-quality ones will be inserted into the corresponding subnet pool. Then, the sampling will gradually be prone to sampling subnets from the subnet pools. Moreover, the one with a better performance metric is assigned with higher priority to train our PSS-Net, if sampling is from a subnet pool. At the end of training, our PSS-Net retains the best subnet in each pool to entitle a fast switch of high-quality subnets for inference when the available resources vary. Experiments on ImageNet using MobileNetV1/V2 show that our PSS-Net can well outperform state-of-the-art resource-adaptive supernets. Our project is at https://github.com/chenbong/PSS-Net.