  The collective behaviour of hadronic particles has been observed in high
multiplicity proton-lead collisions at the Large Hadron Collider (LHC), as well
as in deuteron-gold collisions at the Relativistic Heavy-Ion Collider (RHIC).
In this work we present the first calculation, in the hydrodynamic framework,
of thermal photon radiation from such small collision systems. Owing to their
compact size, these systems can reach temperatures comparable to those in
central nucleus-nucleus collisions. The thermal photons can thus shine over the
prompt background, and increase the low $p_T$ direct photon spectrum by a
factor of 2-3 in 0-1% p+Pb collisions at 5.02 TeV. This thermal photon
enhancement can therefore serve as a clean signature of the existence of a hot
quark-gluon plasma during the evolution of these small collision systems, as
well as validate hydrodynamic behavior in small systems.
