A new and distinct cultivar of Rudbeckia plant named ‘；ET RDB 603’；, characterized by its tall and upright to somewhat outwardly spreading and mounded plant habit； relatively small leaves； dense and bushy growth habit； freely flowering habit； large single-type inflorescences with yellow orange and greyed orange bi-colored ray florets positioned above the foliar plane on strong peduncles； and good postproduction and garden performance.
