The A(2)Pi(g) <- X-2 Pi(u) electronic transition (4800-6000 angstrom) of triacetylene cation was measured in an ion trap, where the vibrational and rotational degrees of freedom were equilibrated to 25 K The rotational profile of the origin band is predicted by a collisional-radiative rate model under conditions expected in diffuse interstellar clouds. Variation in the density of the surrounding gas, rotational temperature, and velocity dispersion are taken into account.