Old Fort Johnson, or Fort Johnson, was a two-story stone house enclosed in fortifications built by Sir William Johnson about 1749 in the town of Amsterdam, Montgomery County, New York, United States. The fort served as Johnson's home, business office and trading center until 1763 when he moved to Johnson Hall in what is now Johnstown, New York. Sir William's son Sir John Johnson owned the house from 1763 until 1776, when it was confiscated by the local .