Ledecka, 22, beat Germany's Selina Joerg in a tense snowboard final. Germany's Ramona Hofmeister took bronze.
The Czech world champion and hot favourite had earlier shocked the world of alpine skiing by capturing gold in last week's super-G skiing.
The men's gold medal went to Nevin Galmarini of Switzerland, who beat South Korea's surprise finalist Lee Sang-ho.
One week ago, Ledecka stunned a strong field including defending champion Anna Veith and American star Lindsey Vonn to win the super-G, becoming one of the stories of the Games.
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With Saturday's win, Ledecka joins Norwegians Thorleif Haug and Johan Groettumsbraten as the only athletes to do a multi-sport double at the Winter Games.
Haug did the cross country and Nordic combined double at the 1924 Olympics and Grottumsbraten won the same events at the 1928 Games.
"She will definitely have a long-lasting impact," American ski star Lindsey Vonn said earlier of Ledecka.
"With the next generation coming up, everyone wants to experience everything. Millennials are raw -- being inclusive and trying other sports is important for them, as it should be.