We study a dark matter (DM) model in which a DM particle interacts predominantly with non- SM particles (mediator particles) which decay into the SM particles later, while the DM particle has a very small coupling to the SM particles. This kind of DM is called secluded DM. We first introduce a simple model of secluded DM to survey the parameter space. Then, it is pointed out that if the mass splitting between DM and mediator is small, there is a novel thermal history of DM in which the DM number density evolution in early universe exhibits a temporary freeze- out behavior. Besides, a larger annihilation cross section than that of well-known thermal relic DM models is required in order to achieve the observed density. Based on these observations, a novel possibility of DM model building is proposed in which DM and mediator are unified in an approximate dark symmetry multiplet. A pionic DM model is introduced to illustrate this idea in a renormalizable field theory framework. The model naturally realizes the degenerate mass spectrum and the large cross section of the DM and the mediator.