There is currently no nationwide system in the U.S. that lets citizens cast a real-time, fully online vote over the internet for federal, state, or local elections. United States election laws and voting methods are determined on a state-by-state basis. While most states do not offer online voting for the general public, exceptions do exist for specific voter groups, such as overseas and military voters, and voters with disabilities.
Note that online voting in the U.S. does not typically refer to a wholly online process, in which voters mark their ballot online and then simply click a button to submit it to their election office. Rather, it typically refers to the ability to electronically receive and return a ballot via email, fax, or an online portal, but voters must still still download, print, and sign their ballot before returning it.
There are exceptions to this in some states where, for instance, voters with disabilities are permitted to sign the voter certification with their finger, mouse, or a stylus, and submit their ballot online.