The present invention relates to an inbred sunflower line, designated 503A/B. The invention relates to the seeds of inbred sunflower line 503A/B, to the plants of inbred sunflower line 503A/B and to the methods for producing a sunflower plant, either inbred or hybrid, by crossing the inbred line 503A/B with itself or another sunflower line. The invention further relates to methods for producing a sunflower plant containing in its genetic material one or more transgenes and to the transgenic plants produced by that method and to methods for producing other inbred sunflower lines derived from the inbred 503A/B.