Excited-state dynamics in poly[4,6-(dodecyl-thieno [3,4-b]thiophene-2-carboxylate)-alt-2,6-(4,8-dioctoxylbenzo [1,24, :4,5-b] dithiophene)] (PTB1) was studied by transient absorption spectroscopy. Upon photoexcitation at 400 nm, an additional transient species is promptly generated along with singlet excitons and survives up to nanoseconds, while singlet excitons disappear completely. In order to assign the long-lived species, we measured transient absorption spectra over the wide spectral range from 900 to 2500 nm. As a result, we found that the long-lived species is ascribed not to polarons but to triplet excitons, which is formed through the ultrafast singlet fission (SF). We discuss the ultrafast SF mechanism in push-pull low-bandgap polymer PTB1 films on the basis of the excited-state dynamics under various excitation wavelengths and intensities.