  The mechanical properties of two-dimensional (2D) materials are important for
a wide range of applications including composite materials, van der Waals
materials and flexible electronics. Several aspects are highly debated in the
literature: For example, the theoretically predicted bending rigidity, $\kappa
(T)$, of the most important 2D material, graphene, varies from 0.8~eV to 10~eV
for predictions at 0 K and there are predictions that it could increase or
decrease with temperature depending on the applied theory. Here we present the
first systematic experimental study of the temperature dependence $\kappa(T)$
of graphene. Extracting values of $\kappa$ from the ZA-phonon dispersion
relation measured with helium atom scattering, we find the value of $\kappa(T)$
to increase with sample temperature. A linear fit in the temperature range (120
K-480 K) gives a bending rigidity of $\kappa (T) = (1.1 \pm 0.1)[\textrm{eV}] +
(0.0013 \pm 0.0001)[\textrm{eV/K}] \times T$.
