The accurate prediction of the optical signatures of cyanine derivatives remains an important challenge in theoretical chemistry. Indeed, up to now, only the most involved quantum chemical methods (CAS-PT2, CC, DMC, etc.) yield consistent and accurate data, impeding the applications on real-life molecules. Here, we investigate the lowest lying singlet excitation energies of increasingly long cyanine dyes within the GW and Bethe−Salpeter Green's function many-body perturbation theory. Our results are in remarkable agreement with available coupled-cluster 17 (exCC3) data, bringing these two single-reference perturbation techniques within a 18 0.05 eV maximum discrepancy. By comparison, available TD-DFT calculations with 19 various semilocal, global, or range-separated hybrid functionals, overshoot the 20 transition energies by a typical error of 0.3−0.6 eV. The obtained accuracy is achieved 21 with a parameter-free formalism that offers similar accuracy for metallic or insulating 22 finite size or extended systems.