The paper reports the detection of 21 cm absorption at the redshift z = 2.03945 + or - 0.00008 in the radio continuum of the QSO PKS 0458-02. This is the largest redshift yet found at radio wavelengths. It corresponds to an optical redshift inferred from damped Ly-alpha and C II, C II(asterisk), and Si IV absorption. The 21 cm spectrum reveals two narrow and deep components. Comparison with the column density inferred from Ly-alpha reveals that the spin temperature is less than 1000 K. It is shown that each component cannot arise in a single pressure-confined cloud. Rather, it is suggested that the absorption site is a highly redshifted galactic disk, or possibly two isolated, self-gravitating clouds.