Williamson, a Democrat who ended her own bid for the nomination in January stating she didn't want to "get in the way of a progressive candidate winning" the early primary and caucuses, previously had supported the self-proclaimed Democratic socialist Sanders in his first run for president in 2016.
Williamson made the announcement in her home state of Texas, at a Sanders rally in Austin.
Sanders picked up victories in all three of the earliest state contests, most recently clinching Nevada and solidifying his status as the party's frontrunner.
"It's time for us to take a stand with Bernie," Williamson said at the event. "It's our turn now."
She later went on to release a statement saying of Sanders "I am honored to endorse him."
"What happened in Nevada on Saturday was extraordinary and the energy is unquestionable with Bernie," the statement read, pointing to Sanders' nearly 40-percent lead in the state's caucuses. "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. That revolution is one that Bernie Sanders now leads."
The remaining primary contenders are moving on to South Carolina's contest this coming Saturday, the last of the primaries before Super Tuesday.
Fox News' Andrew Craft in Austin, Texas, and The Associated Press contributed to this report.