Spin transport and manipulation in semiconductors have been studied intensively with the ultimate goal of realizing spintronic devices. Previous work in GaAs has focused on controlling the carrier density1, crystallographic orientation(2) and dimensionality(3,4) to limit the electron spin decoherence and allow transport over long distances(4-7). Here, we introduce a new method for the coherent transport of spin-polarized electronic wave packets using dynamic quantum dots (DQDs) created by the piezoelectric field of coherent acoustic phonons(8-11). Photogenerated spin carriers transported by the DQDs in undoped GaAs (001) quantum wells exhibit a spin coherence length exceeding 100 mu m, which is attributed to the simultaneous control of the carrier density and the dimensionality(12) by the DQDs during transport. In the absence of an applied magnetic field, we observe the precession of the electron spin induced by the internal magnetic field associated with the spin splitting of the conduction band (Dresselhaus term)(13). The coherent manipulation of the precession frequency is also achieved by applying an external magnetic field.