A new catecholamine polymer, tannin-polyethyleneimine (TA-PEI) was fabricated via mussel-inspired polymerization for highly efficient hexavalent chromium (Cr(VI)) adsorption. The TA-PEI adsorbent was prepared with a mass ratio of TA to PEI of 3:1. This adsorbent had an ultra-high Cr(VI) removal capacity because of its amines and phenolic hydroxyl groups, with values reaching 707.96 mg/g under 308 K, which is higher than that of most PEI-based and TA-bearing adsorbents. The developed adsorbent also achieved rapid Cr(VI) adsorption, with adsorption capacity values reaching 554.94 mg/g in 15 min. Further, it displayed excellent chemical reduction ability when converting Cr(VI) to low toxicity Cr(III), as well as its good adsorption selectivity of Cr(VI) in a solution bearing coexisting ions. In a continuous adsorption experiment, the adsorbent exhibited a Cr(VI) solution treatment capacity of up to 30 L/g with a Cr(VI) concentration of 10 ppm. With its high adsorption capacity and rapid adsorption rate, this low cost and easily prepared adsorbent may have widespread potential for Cr(VI) removal and enrichment.