Real quantum systems always interact with the environment. This interaction leads to decoherence, the process by which quantum information is degraded and purely quantum properties of a system are lost. Decoherence provides a theoretical basis for the quantum- classical transition [1], emerging as a possible explanation of the quantum origin of the classical world. It is also a major obstacle for building a quantum computer [2] since it can produce the loss of the quantum character of the computer. Therefore, a complete characterization of the decoherence process and its relation with the physical properties of the system and the environment is needed for both fundamental and practical purposes. The connection between decoherence and environmental quantum phase transitions has been recently investigated [3–5]. A universal Gaussian decay regime in the fidelity of the system was initially identified, and related to a second-order quantum phase transition in the environment [4]; as a consequence, the decoherence process was postulated as an indicator of a quantum phase transition in the environment. Subsequently, this analysis was refined, and it was found that the universal regime is neither always Gaussian [6], nor always related to an environmental quantum phase transition [5]. In this paper we analyze the relationship between decoherence and an environmental excited state quantum phase transition (ESQPT). We show that the fidelity of a single qubit, coupled to a two-level boson environment, becomes singular when the system-environment coupling energy equals the critical energy for the occurrence of a continuous ESQPT in the environment. Therefore, our results establish that a critical phenomenon in the environment entails a singular behavior in the decoherence induced in the central system. An ESQPT is a nonanalytic evolution of some excited states of a system as the Hamil- tonian control parameter is varied. It is analogous to a standard quantum phase transition (QPT), but taking place in some excited state of the system, which defines the critical en- ergy E$_{c}$ at which the transition takes place. We can distinguish between different kinds of ESQPT. As it is stated in [7], in the thermodynamic limit a crossing of two levels at E = E$_{c}$ determines a first order ESQPT, while if the number of interacting levels is locally large at E = E$_{c}$ but without real crossings, the ESQPT is continuous. In this paper we will concentrate in the latter case, which usually entails a singularity in the density of states (for an illustration see Fig. 1). As the entropy of a quantum system is related to its density of states, a relationship between an ESQPT and a standard phase transition at a certain critical temperature, can be established in the thermodynamic limit [8]. 2