The concentration of atomic hydrogen in the Venus thermosphere near 165 km altitude and ~18¿ north latitude has been derived from Pioneer Venus in situ measurements of n(H+), n(O+), n(O), and n(CO2), under the assumption of chemical equilibrium. Altitude profiles of derived n(H) suggest that chemical equilibrium prevails to an altitude of at least 200 km on the dayside and to 165 km on the nightside. Measurements below these limits were made by the ion and neutral mass spectrometers on the orbiter spacecraft between December 1978 and July 1979, while periapsis traversed a complete diurnal cycle. The hydrogen concentration is found to rise sharply at both terminators from a dayside value of ~5¿104 cm-3, and to exhibit an asymmetric nightside distribution with a peak density in the predawn sector approximately 400 times greater than the dayside value. Analysis suggests that wind-induced diffusion, combined with exospheric return flow, can account for the observed hydrogen behavior. The large day-night temperature contrast enhances advective transport, which produces the large n(H) diurnal variation; the shift of the n(H) nighttime maximum toward dawn is caused by atmospheric superrotation.