  In this paper we explicitly carry out the perturbative renormalization of the
$T\bar{T}$-deformed free massive Dirac fermion in two dimensions up to second
order in the coupling constant. This is done by computing the two-to-two
$S$-matrix using the LSZ reduction formula and canceling out the divergences by
introducing counterterms. We demonstrate that the renormalized Lagrangian is
unambiguously determined by demanding that it gives the correct $S$-matrix of a
$T\bar{T}$-deformed integrable field theory. Remarkably, the renormalized
Lagrangian is qualitatively very different from its classical counterpart.
