Néstor Guillén Olmos (28 January 1890 – 12 March 1966) was a Bolivian lawyer and politician who served as the 40th president of Bolivia on a de facto interim basis in 1946.
Born in La Paz, Guillén studied law and rose to become the dean of the Superior District Court of La Paz.
Néstor Guillén served as President of Bolivia for 27 days between July and August 1946, following the overthrow and assassination of President Gualberto Villarroel (1943–46). Upon the death of Villarroel, the alliance of forces that had toppled him needed a reliable and impartial caretaker (given the mood of the citizenry, which had just shown what it was capable of during the revolt) to guide the country to elections. They settled on the head of the La Paz Court of Appeals, Tomás Monje Gutiérrez, who was ill at the time. Thus, Guillén filled in for him for fewer than four weeks, whereupon Monje was sworn-in as President. The old oligarchy was established again but, not long after, would fall only a few years later.
Guillén then returned to his judgeship and died on 12 March 1966 in La Paz, Bolivia.