The phosphorus nuclear spin‐lattice relaxation rate 1/T1has been measured in the quasi‐two‐dimensionalXYantiferromagnet BaNi2(PO4)2. The divergence observed just above the ordering temperatureTN&bartil; 23.6 K is analyzed within the context of the Kosterlitz–Thouless transition. For a magnetic fieldHperpendicular to theXYplane, the data presented here agree well with a recent model of freely moving vortices. The field dependence observed forHparallel suggests a drastic slowing down of the vortex velocity in that case.