Measuring the hydrogen-deuterium isotope shift via two-photon spectroscopy of the 1S-2S transition, we obtain 670,994,334,606(15) Hz. This is a 10-times improvement over the previous best measurement [A. Huber, Phys. Rev. Lett. 80, 468 (1998)] confirming its frequency value. A calculation of the difference of the mean square charge radii of deuterium and hydrogen results in <r2>d - <r2>p =3.82007(65) fm2, a more than twofold improvement compared to the former value.