Multiconfigurational quantum chemical methods (CASSCF/CASPT2) have been used to study the chemical bond in the actinide diatoms Ac-2, Th-2, Pa-2, and U-2. Scalar relativistic effects and spin-orbit coupling have been included in the calculations. In the Ac-2 and Th-2 diatoms the atomic 6d, 7s, and 7p orbitals are the significant contributors to the bond, while for the two heavier diatoms, the 5f orbitals become increasingly important. Ac-2 is characterized by a double bond with a (3)Sigma(-)(g)(0(g)(+)) ground state, a bond distance of 3.64. angstrom, and a bond energy of 1.19 eV. Th-2 has quadruple bond character with a D-3(g)(1(g)) ground state. The bond distance is 2.76 angstrom and the bond energy (D-0) 3.28 eV. Pa-2 is characterized by a quintuple bond with a (3)Sigma(-)(g)(0(g)(+)) ground state. The bond distance is 2.37 angstrom and the bond energy 4.00 eV. The uranium diatom has also a quintuple bond with a O-7(g) (8(g)) ground state, a bond distance of 2.43 angstrom, and a bond energy of 1.15 eV. It is concluded that the strongest bound actinide diatom is Pa-2, characterized by a well-developed quintuple bond.