We report the deterministic growth of individual single-crystal organic semiconductor nanowires of silver-tetracyanoquinodimethane (Ag-TCNQ) with high yield (>90%) by a vapor-solid chemical reaction process. Ag-metal films or patterned dots deposited onto substrates serve as chemical reaction centers and are completely consumed during the growth of the individual or multiple nanowires. Selective-area electron diffraction (SAED) revealed that the Ag-TCNQ nanowires grow preferentially along the strong pi-pi stacking direction of Ag-TCNQ molecules. The vapor-solid chemical reaction process described here permits the growth of organic nanowires at lower temperatures than chemical vapor deposition (CVD) of inorganic nanowires. The single-crystal Ag-TCNQ nanowires are shown to act as memory switches with high on/off ratios, making them potentially useful in optical storage, ultrahigh-density nanoscale memory, and logic devices.