Catharanthus roseus (L.) G. Don (Apocynaceae) is a medicinal plant that produces indole alkaloids used in cancer chemotherapy. Commercially important antineoplastic alkaloids, namely, vinblastine and vincristine, are mainly present in the leaves of C. roseus. Gamma-rays irradiated carrageenan (ICR) has been proven as plant growth promoting substance for a number of medicinal and agricultural plants. Considering the importance of ICR as a promoter of plant growth and alkaloids production in C. roseus, a pot experiment was carried out to explore the effect of ICR on the plant growth, physiological activities, and production of anticancer alkaloids in C. roseus at 120 and 150 days after planting (DAP). Foliar application of ICR (at 0, 20, 40, 60, 80, and 100 mg L-1) significantly improved the performance of C. roseus. 80 mg L-1 of ICR enhanced the leaf yield by 29.2 and 35.4% and the herbage yield by 32.5 and 37.4% at 120 and 150 DAP, respectively, over the control. The spray of ICR at 80 mg L-1 increased the yield of vinblastine by 64.3 and 65.0% and of vincristine by 75.5 and 77.0% at 120 and 150 DAP, respectively, as compared to the control.