Context. WASP-127b is a planet with one of the lowest densities discovered to date. With a sub-Saturn mass (Mp = 0.18 ± 0.02 MJ) and super-Jupiter radius (Rp = 1.37 ± 0.04 RJ), it orbits a bright G5 star that is about to leave the main-sequence. Aims: We aim to explore the atmosphere of WASP-127b in order to retrieve its main atmospheric components, and to find hints for its intriguing inflation and evolutionary history. Methods: We used the ALFOSC spectrograph at the NOT telescope to observe a low-resolution (R 330, seeing limited) long-slit spectroscopic time series during a planetary transit, and present here the first transmission spectrum for WASP-127b. Results: We find a strong Rayleigh slope at blue wavelengths and a hint of Na absorption, although the quality of the data does not allow us to claim a detection. At redder wavelengths the absorption features of TiO and VO are the best explanation to fit the data. Conclusions: Although observations with a higher signal-to-noise ratio are needed to conclusively confirm the absorption features, WASP-127b seems to posses a cloud-free atmosphere and is one of the best targets on which to perform further characterization studies in the near future.