  Primordial magnetic fields lead to non-Gaussian signals in the Cosmic
Microwave Background (CMB) even at the lowest order, as magnetic stresses, and
the temperature anisotropy they induce, depend quadratically on the magnetic
field. In contrast, CMB non-Gaussianity due to inflationary scalar
perturbations arise only as a higher order effect. We propose here a novel
probe of stochastic primordial magnetic fields that exploits the characteristic
CMB non-Gaussianity that they induce. In particular, we compute the CMB
bispectrum ($b_{l_{_1}l_{_2}l_{_3}}$) induced by stochastic primordial fields
on large angular scales. We find a typical value of $l_1(l_1+1)l_3(l_3+1)
b_{l_{_1}l_{_2}l_{_3}} \sim 10^{-22}$, for magnetic fields of strength $B_0
\sim 3$ nano Gauss and with a nearly scale invariant magnetic spectrum. Current
observational limits on the bispectrum allow us to set upper limits on $B_0
\sim 35$ nano Gauss, which can be improved by including other magnetically
induced contributions to the bispectrum.
