Catharanthus roseus is a medicinal plant that produces indole alkaloids, which are utilized in anticancer therapy. Vinblastine and vincristine, two commercially important antineoplastic alkaloids, are mostly found in the leaves of Catharanthus roseus. ĸ-carrageenan has been proven as plant growth promoting substance for a number of medicinal and agricultural plants. Considering the importance of ĸ-carrageenan as a promoter of plant growth and phytochemical constituents, especially alkaloids production in Catharanthus roseus, an experiment was carried out to explore the effect of ĸ-carrageenan on the plant growth, phytochemicals content, pigments content, and production of antitumor alkaloids in Catharanthus roseus after planting. Foliar application of ĸ-carrageenan (at 0, 400, 600 and 800 ppm) significantly improved the performance of Catharanthus roseus. Phytochemical analysis involved determining the amount of total phenolics (TP), flavonoids (F), free amino acids (FAA), alkaloids (TAC) and pigments contents by spectrophotometer, minerals by ICP, amino acids, phenolic compounds and alkaloids (Vincamine, Catharanthine, Vincracine (Vincristine), and vinblastine) analysis uses HPLC. The results indicated that all examined ĸ-carrageenan treatments led to a significant (p ≤ 0.05) increase in growth parameters compared to the untreated plants. Phytochemical examination indicates that the spray of ĸ-carrageenan at 800 mg L−1 increased the yield of alkaloids (Vincamine, Catharanthine and Vincracine (Vincristine)) by 41.85 μg/g DW, total phenolic compounds by 3948.6 μg gallic/g FW, the content of flavonoids 951.3 μg quercetin /g FW and carotenoids content 32.97 mg/g FW as compared to the control. An amount of 400 ppm ĸ-carrageenan treatment gave the best contents of FAA, Chl a, Chl b and anthocyanin. The element content of K, Ca, Cu, Zn and Se increased by treatments. Amino acids constituents and phenolics compounds contents were altered by ĸ-carrageenan.