  We calculate tensor bispectrum in a theory of massive tensor gravitons which
predicts blue-tilted and largely amplified primordial gravitational waves. We
find that a new 3-point interaction can produce a larger tensor bispectrum than
the conventional one from general relativity. The bispectrum peaks at the
squeezed configuration and its slope towards the squeezed limit is determined
by the graviton mass. This squeezed tensor bispectrum may be observed as the
quadrupolar modulation of the tensor power spectrum by interferometers.
