Gassendi was born at Champtercier in France. After being educated in Aix and Paris, he gained a doctorate in theology from Avignon in 1616, was ordained in 1617, and in the same year was appointed to the chair of philosophy at Aix. In 1624 Gassendi moved to Digne where he served as provost of the cathedral until 1645 when he was elected to the professorship of mathematics at the Collège Royale in Paris, resigning because of illness in 1648.

As a practicing astronomer Gassendi made a large number of observations of comets, eclipses, and such celestial phenomena as the aurora borealis – a term he introduced himself. His most significant observation was of the 1631 transit of Mercury, the first transit to be observed, which he recorded in his Mercurius in sole visus (1632; Mercury in the Face of the Sun) as support for the new astronomy of Johannes Kepler.

In physics Gassendi attempted to measure the speed of sound and obtained the (too high) figure of 1473 feet per second. He also, in 1640, performed the much contemplated experiment of releasing a ball from the mast of a moving ship; as he expected, it fell to the foot of the mast in a straight line.

Gassendi's importance to science rests with his role as a propagandist and philosopher rather than as an experimentalist. Even though the Paris parliament declared in 1624 that on penalty of death “no person should either hold or teach any doctrine opposed to Aristotle,” Gassendi published in the same year his Excertitationes…adversus Aristoteleos (Dissertations…against Aristotle), the first of his many works attacking both medieval Scholasticism and Aristotelianism. Nor did Gassendi find much attraction in the then emerging system of René Descartes. Instead he sought in his influential Animadversiones in decimum librum Diogenes Laertii (1649; Observations on the Tenth Book of Diogenes Laertius) to revive the classical atomism of Epicurus, suitably modified to ensure its compatibility with 17th-century Christianity. Unlike Epicurus he insisted that the atoms were created by God who also bestowed on man an immaterial soul; against Descartes he admitted the existence of the void within which his atoms could interact.

Gassendi's works were well known in England and exercised considerable influence on such leading scientists as Robert Boyle.