  By explicit evaluation of certain disk S-matrix elements in the presence of
background B-flux, we find coupling of two open string tachyons to gauge field,
graviton, dilaton or Kalb-Ramond antisymmetric tensor on the world-volume of a
single non-BPS D$p$-brane. We then propose an extension of the abelian
Dirac-Born-Infeld action which naturally reproduces these couplings in field
theory. This action includes non-linearly the dynamics of the tachyon field
much like the other bosonic modes of the non-BPS D$p$-brane. On the general
grounds of gauge and T-duality transformations and the symmetrized trace
prescription, we then extend the abelian action to non-abelian cases.
