To confirm the capability and mechanisms of Sr(II) and Cs(I) adsorption from the aqueous phase using basil seed (BS), virgin BS, calcined BS (BS500 and BS1000), and enzymatically treated BS, namely Mannanase BGM (M-BS), Pectinase G (P-BS), Hemicellulase (H-BS), and Cellulase A (C-BS) was evaluated. The adsorption capabilities of Sr(II) and Cs(I) of various BS adsorbents were also evaluated. The quantity of Sr(II) and Cs(I) adsorbed onto BS was greater than that of BS500 or BS1000, suggesting that the physicochemical characteristics of the BS surface affected Sr(II) and Cs(I) removal from the aqueous phase. Furthermore, the quantity of Sr(II) and Cs(I) adsorbed onto virgin BS was greater than that of enzymatically treated BS, indicating that glucomannan or (1,4)-xylan in the cellulosic hydrocolloid of the BS strongly affected the adsorption capability of Cs(I) or Sr(II) (except for M-BS in Sr(II) adsorption). Our obtained results indicate that, as an adsorbent, BS was capable of removing Sr(II) and Cs(I) from the aqueous solution.