Geminiano Montanari (1 June 1633 – 13 October 1687) was an Italian astronomer, lens-maker, and proponent of the experimental approach to science.

Montanari's famous students include Domenico Guglielmini, Francesco Bianchini, Gianantonio Davia and Luigi Ferdinando Marsili.

He is best known for his observation, made around 1667, that the second-brightest star (called Algol as derived from its name in Arabic) in the constellation of Perseus varied in brightness.

The star's names in Arabic, Hebrew and other languages, all of which have a meaning of "ghoul" or "demon", imply that its unusual behaviour had long been recognised.

Thanks to the influence of Paolo del Buono, one of Galileo's last direct disciples and Florentine diplomat at the imperial court, he pursued the mathematical studies begun in Modena at the age of thirteen.

In the meantime the Modenese scientist had made acquaintance with Marquis Cornelio Malvasia, an influential senator and patron of science of Bologna who had built in his country house near Modena an astronomical observatory.

Malvasia also managed to get his protégé a chair of mathematics at the University of Bologna, to which Montanari was appointed by the Senate in December 1664.

In the period shortly after Galileo Galilei, experimentalists like Montanari were engaged in a battle against the more mystical views of scientists such as Donato Rossetti, a pupil of Borelli.

In his researches, says Graziani, Montanari succeeds in explaining all the general phenomena of value, though without thoroughly understanding the intricate and difficult subject – the value of money.