The author made the announcement Sunday during a surprise appearance at a rally in Austin, Texas, for the Vermont senator.
"Bernie Sanders has taken a stand, and Bernie Sanders has been taking a stand for a very long time," she told the crowd. "He has been consistent, he has been convicted, he has been committed -- And now it's time. I'm here and you're here because it's time for us to take a stand with Bernie."
Williamson, who dropped out of the race for the Democratic nomination in January, had previously thrown her support behind businessman Andrew Yang before he, too, pulled his name from contention earlier this month following a poor showing in the New Hampshire primaries.
Williamson's endorsement follows Sanders overwhelming the competition in the Nevada caucuses, gaining 47.1 percent of delegate equivalents with 87 percent of precincts reporting by late Sunday.
In a statement published on Twitter Sunday, Williamson said his Nevada victory was "extraordinary."
"A 40-year-old trend of capitalism without conscience -- corporate elites and their errand boys in government -- have created the inevitable blowback in the form of a political revolution," her statement read. "That revolution is one that Bernie Sanders now leads, and I want to do everything I can to further its success. I am honored to endorse him."