  The impact of QCD instantons on scalar glueball properties is studied in the
framework of an instanton-improved operator product expansion (IOPE) for the
0^{++} glueball correlation function. Direct instanton contributions are found
to strongly dominate over those from perturbative fluctuations and soft vacuum
fields. All IOPE sum rules, including the one involving a subtraction constant,
show a high degree of stability and are, in contrast to previous glueball sum
rules, consistent with the low-energy theorem for the zero-momentum correlator.
The predicted glueball mass m_{G}=1.53 \pm 0.2 GeV is less sensitive to the
instanton contributions then the glueball coupling (residue) f_{G}=1.01\pm 0.25
GeV, which increases by about half an order of magnitude. Both glueball
properties are shown to obey scaling relations as a function of the average
instanton size and density.
