We study celestial amplitudes in string theory at one-loop. Celestial
amplitudes describe scattering processes of boost eigenstates and relate to
amplitudes in the more standard basis of momentum eigenstates through a Mellin
transform. They are thus sensitive to both the ultraviolet and the infrared,
which raises the question of how to appropriately take the field theory limit
of string amplitudes in the celestial basis. We address this problem in the
context of four-dimensional genus-one scattering processes of gluons in open
string theory which reach the two-dimensional celestial sphere at null
infinity. We show that the Mellin transform commutes with the adequate limit in
the worldsheet moduli space and reproduces the celestial one-loop field theory
amplitude expressed in the worldline formalism. The dependence on $\alpha'$
continues to be a simple overall factor in one-loop celestial amplitudes albeit
with a power that is shifted by three units with respect to tree-level, thus
making manifest that the dimensionless parameter $g_{10}^2/\alpha'^3$ organizes
the loop expansion in the celestial basis.
