Organoborane acceptor-modified polythiophene has been prepared from silylated polythiophene via a new polymer modification strategy. The attachment of electron-deficient dimesitylboryl groups leads to a significant decrease of the LUMO levels as evidenced by a strong bathochromic shift in the UV-visible absorption and emission spectra and the observation of reversible reduction waves in the cyclic voltammogram at significantly less cathodic potentials than for the silylated precursor polymer. The modification of conjugated polymers with electron-deficient boryl groups provides a new design principle for the preparation of electronically interesting materials.