We study the entanglement between a qubit and its environment from the spin-boson model with Ohmic dissipation. Through a mapping to the anisotropic Kondo model, we derive the entropy of entanglement of the spin E(α, ∆, h), where α is the dissipation strength, ∆ is the tunneling amplitude between qubit states, and h is the level asymmetry. For 1 − α ≫ ∆/ωc and (∆, h) ≪ ωc, we show that the Kondo energy scale TK controls the entanglement between the qubit and the bosonic environment (ωc is a high-energy cutoff). For h ≪ TK, the disentanglement proceeds as (h/TK ) 2 ; for h ≫ TK, E vanishes as (TK/h) 2−2α , up to a logarithmic correction. For a given h, the maximum entanglement occurs at a value of α which lies in the crossover regime h ∼ TK . We emphasize the possibility of measuring this entanglement using charge qubits subject to electromagnetic noise. The concept of quantum entropy appears in multiple contexts, from black hole physics 1 to quantum information theory, where it measures the entanglement of quantum states.2 Prompted by the link between entanglement and quantum criticality, 3 a number of researchers have begun to study the entanglement entropy of condensed matter systems. In this Letter, we employ the spin-boson model 4,5 to describe the entanglement between a qubit (two-level system) and an infinite collection of bosons. With an Ohmic bosonic bath, the spin-boson model undergoes a quantum phase transition of Kosterlitz-Thouless type when α − 1 = ∆/ω c , where α is the strength of the coupling to the environment, ∆ is the tunneling amplitude between the qubit states, and ω c ≫ ∆ is an ultraviolet cutoff.6,7 When the two levels of the qubit are degenerate, the entanglement between the qubit and the bosons is discontinuous at this transition.8,9 Here we report the first rigorous analytical results for the entanglement (quantum entropy) in the strongly entangled regime 1 − α ≫ ∆/ω c .We exploit a mapping between the spin-boson model and the anisotropic Kondo model; our results follow from the Bethe ansatz solution of the equivalent interacting resonant level model. 10,11 We show that the entropy of entanglement (E) of the qubit with the environment is controlled by the Kondo energy scale T K