Néstor Guillén Olmos (28 January 1890 – 12 March 1966) was a Bolivian judge, lawyer, and politician who served as the 40th president of Bolivia from July to August 1946.

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.

Guillén then returned to his judgeship and died on 12 March 1966 in La Paz, Bolivia.