An efficient plant regeneration protocol using cotyledon explants was established for castor (Ricinus communis L.), an important oilseed crop. Mature seed-derived cotyledon explants produced adventitious shoots when placed on Murashige and Skoog (MS) medium containing thidiazuron (TDZ). The rate of shoot regeneration was maximal (≈25 shoots per explant) when explants were cultured on shoot induction medium supplemented with 5 μm TDZ and preincubated in the dark for the first 7 days before transferring to the day/night cycle (16/8 h). Only the proximal ends of cotyledon explants produced adventitious shoots, although green calli were observed in cotyledon veins. After 4 weeks in culture, explants with well-developed shoot buds were transferred to MS medium without plant growth regulators for the shoot elongation and development. At ≈4 months after culture initiation, shoots (2 cm in length) were transferred to root induction medium (MS medium supplemented with 5 μm indole-3-butyric acid) where they developed roots in 4 to 6 weeks. Plantlets were transferred to soil and acclimatized to greenhouse conditions. Histological analysis showed the adventitious induction of the shoots originated from the cortical and epidermal cell layers of the cotyledon explants.