Context. Star formation rate (SFR), metallicity, and stellar mass are among the most important parameters of star-forming (SF) galaxies characterizing their formation and evolution. They are known to be related to each other both at low and high redshift in the mass-metallicity, mass-SFR, and metallicity-SFR relations. Aims: We demonstrate the existence of a plane in a 3D parameter space defined by the axes SFR [log (SFR)(M&sun; yr-1)] , gas metallicity [12 + log (O/H)], and stellar mass [log (Mstar/M&sun;)] of SF galaxies. Methods: We used SF galaxies from the “main galaxy sample” of the Sloan Digital Sky Survey-Data Release 7 (SDSS-DR7) in the redshift range 0.04 < z < 0.1 and r-magnitudes between 14.5 and 17.77. Metallicities, SFRs, and stellar masses were taken from the Max-Planck-Institute for Astrophysics-John Hopkins University (MPA-JHU) emission-line analysis database. Results: From a final sample of 32 575 galaxies, we find for the first time a fundamental plane for field galaxies relating the SFR, gas metallicity, and stellar mass for SF galaxies in the local universe. One of the applications of this plane would be to estimate stellar masses from SFR and metallicity. High redshift data from the literature at redshift ~0.85, 2.2, and 3.5, do not show evidence of evolution in this fundamental plane. Appendix A is only available in electronic form at http://www.aanda.org