Background Androgen ablation therapy is the primary treatment for metastatic prostate cancer (PCa). However, the majority of PCa patients receiving the androgen deprivation therapy develop recurrent castration-resistant prostate cancer (CRPC) within two years. Chemotherapies show little effect on prolonging survival of CRPC patients and new treatments are needed. Previous studies reported that the extracts from rooibos (Aspalathus linearis) exhibit chemopreventive properties in some cancer models, including skin, liver and oesophagus cancers in animals. We therefore investigate if extracts from rooibos can suppress the proliferation of CRPC cells. Purpose We investigated whether an aspalathin-rich green rooibos extract (GRT™; 12.78 g aspalathin/100 g extract) demonstrates anti-cancer activity against CRPC cells. Methods High performance liquid chromatography (HPLC) was used to profile the major flavonoids in GRT. Hoechst-dye proliferation assay, 3,4,5-dimethylthiazol-2-yl)-2-5-diphenyltetrazolium bromide (MTT) viability assay and flow cytometry assay were used to explore the effects of GRT on the proliferation and cell cycle progression of CRPC cells. Comet assay was used to survey whether GRT induces apoptosis in CRPC cells. LNCaP 104-R1 xenograft nude mice model was used to determine the inhibitory effect of GRT on CRPC tumors in vivo. Micro-Western Array (MWA) and Western blot analysis were carried out to unravel the underlying molecular mechanism. Results GRT contained aspalathin as the most abundant flavonoid. GRT suppressed the proliferation and survival of LNCaP 104-R1, LNCaP FGC and PC-3 PCa cells. Flow cytometry analysis showed that GRT decreased the population of PCa cells in S phase but increased the cell population in G2/M phase. Comet assay confirmed that GRT induced apoptosis in LNCaP 104-R1 cells. Gavage of 400 mg/kg GRT suppressed LNCaP 104-R1 xenografts in castrated nude mice. MWA and Western blot analysis indicated that GRT treatment suppressed Akt1, phospho-Akt Ser473, Cdc2, Bcl-2, TRAF4 and Aven, but increased activated Caspase 3, cytochrome c, and p27Kip1. Overexpression of Akt rescued the suppressive effects of GRT on CRPC cells. Co-treatment of GRT with Bcl-2 inhibitor ABT-737, PI3K inhibitor LY294002 and Akt inhibitor GSK 690693 exhibited additive inhibitory effect on proliferation of CRPC cells. Conclusions GRT suppresses the proliferation of CRPC cells via inhibition of Akt signaling.