  We study the Lamb shift of a two-level atom arising from its coupling to the
conformal massless scalar field, which satisfies the Dirichlet boundary
conditions, in the Hartle-Hawking vacuum in the BTZ spacetime, and find that
the Lamb shift in the BTZ spacetime is structurally similar to that of a
uniformly accelerated atom near a perfectly reflecting boundary in
(2+1)-dimensional flat spacetime. Our results show that the Lamb shift is
suppressed in the BTZ spacetime as compared to that in the flat spacetime as
long as the transition wavelength of the atom is much larger than $AdS$ radius
of the BTZ spacetime while it can be either suppressed or enhanced if the
transition wavelength of the atom is much less than $AdS$ radius. In contrast,
the Lamb shift is always suppressed very close to the horizon of the BTZ
spacetime and remarkably it reduces to that in the flat spacetime as the
horizon is approached although the local temperature blows up there.
