R package synthACS: Synthetic Microdata and Spatial MicroSimulation Modeling for ACS Data. Firstly provides a wrapper to library(acs) to access numerous American Community Survey (ACS) base tables which may be of interest to many researchers. Secondly, it builds synthetic micro-datasets of ACS data at any specified geographic level with 10 default individual attributes. Thirdly, the package provides functionality for users to add additional ACS & non-ACS data attributes to micro-datasets based on macro population characteristics. And finally, the package also conducts spatial microsimulation modeling (SMSM) via simulated annealing. SMSM is conducted in parallel by default.