We report an experimental determination of the diamagnetic correction to the 9 Be + ground state hyperfine constant A. We measured A = −625 008 837.371(11) Hz at a magnetic field B of 4.4609 T. Comparison with previous results, obtained at lower values of B (0.68 T and 0.82 T), yields the diamagnetic shift coefficient k = 2.63(18) × 10 −11 T −2 , where A(B) = A 0 (1 + kB 2 ). The zero-field hyperfine constant A 0 is determined to be −625 008 837.044(12) Hz. The g-factor ratio g I /g J is determined to be 2.134 779 852 7(10) × 10 −4 , which is equal to the value measured at lower B to within experimental error. Upper limits are placed on some other corrections to the Breit-Rabi formula. The measured value of k agrees with theoretical estimates.