  We calculate the scalar form factor of the pion using two dynamical flavors
of non-perturbatively $\mathcal{O}(a)$-improved Wilson fermions, including both
the connected and the disconnected contribution to the relevant correlation
functions. We employ the calculation of all-to-all propagators using stochastic
sources and a generalized hopping parameter expansion. From the form factor
data at vanishing momentum transfer, $Q^2=0$, and two non-vanishing $Q^2$ we
obtain an estimate for the scalar radius $\left<r^2\right>^\pi_{_{\rm S}}$ of
the pion at one value of the lattice spacing and for five different pion
masses. Using Chiral Perturbation Theory at next-to-leading order, we find
$\left<r^2\right>^\pi_{_{\rm S}}=0.635\pm0.016$ fm$^2$ at the physical pion
mass (statistical error only). This is in good agreement with the
phenomenological estimate from $\pi\pi$-scattering. The inclusion of the
disconnected contribution is essential for achieving this level of agreement.
