A three-dimensional black hole solution of Einstein equations with a n
egative cosmological constant coupled to a conformal scalar field is g
iven. The solution is static, circularly symmetric, asymptotically ant
i-de Sitter-type and nonperturbative in the conformal field. The curva
ture tensor is singular at the origin while the scalar field is regula
r everywhere. The condition that the Euclidean geometry be regular at
the horizon fixes the temperature to be T=9 r(+)/16 pi l(2). Using the
Hamiltonian formulation including boundary terms of the Euclidean act
ion, the entropy is found to be 2/3 of the standard value (1/4A), and
in agreement with the first law of thermodynamics.