Lanthanum (La(Ⅲ)) is known to alleviate heavy metal stress. However, the bioavailability of inorganic La(Ⅲ) is limited due to easy oxidization and low absorption. This study synthesized and characterized the lanthanum(Ⅲ)-amino acid chelate (La(Ⅲ)-AA) from soybean protein isolate (SPI) hydrolysates. Maximum chelating rate (94.95%) was obtained at mole ratio 1:1.5, pH 8.0, 50 ℃ and 5 h. Glu, Asp and Pro represent the primary La(Ⅲ)-binding ligands. UV-vis and FTIR demonstrated that amino nitrogen and carboxyl oxygen participate in metal-ligand recognition. Scanning and Transmission electron microscopy showed that La(Ⅲ) chelates with amino acids in a core-shell structure of uniform size. Based on this, putative chemical structure of La(Ⅲ)-AA was suggested. La-AA outperforms inorganic La salts in growth promotion and Cu detoxification. This study provides a novel La(Ⅲ)-based candidate for crop protection and advances our knowledge of rare earth-induced amelioration in heavy metal stress.