Philipp Eduard Anton von Lenard (German: [ˈfɪlɪp ˈleːnaʁt] ^ⓘ; 7 June 1862 – 20 May 1947) was a Hungarian–German experimental physicist who received the Nobel Prize in Physics in 1905 for his work on cathode rays. This work led to his experimental realization of the photoelectric effect, discovering that the energy (speed) of the electrons ejected from a cathode depends only on the frequency, and not the intensity of light.

As an active proponent of the Nazi ideology, Lenard supported Adolf Hitler in the 1920s and was an important role model for the Deutsche Physik movement during the Nazi period. Notably, he labeled Albert Einstein's contributions to theoretical physics as "Jewish physics".

Philipp Eduard Anton von Lenard was born on 7 June 1862 in Pressburg (now Bratislava, Slovakia), then located in the Kingdom of Hungary, the son of Philipp von Lenard (1812–1896), a wine merchant in Pressburg, and Antonie Baumann (1831–1865). His father's family had originally come from Tyrol, while his mother's family originated from Baden; both parents were German-speaking. Among his mostly Germanic ancestors, he also had Hungarian ones.

Lenard attended the Pozsonyi Királyi Katolikus Főgymnasium (today Gamča), and as he records in his autobiography, this made a big impression on him (especially the personality of his teacher, Virgil Klatt). In 1880, he studied physics and chemistry in Vienna and in Budapest. In 1882, he left Budapest and returned to Pressburg, but in 1883 moved to Heidelberg after his tender for an assistant's position at the University of Budapest was refused. At the University of Heidelberg, he studied under Robert Bunsen, interrupted by one semester in Berlin with Hermann von Helmholtz. He also studied under Georg Quincke, and received his Ph.D. in 1886. The following year, he worked as a demonstrator under Loránd Eötvös at Budapest.

In 1892, Lenard became a Privatdozent and an assistant to Heinrich Hertz at the University of Bonn; Lenard and Hertz conducted experiments with cathode rays, which led to him winning the 1905 Nobel Prize in Physics. After brief stays at Breslau (1894–1895) and Aachen (1895–1896), he was appointed Professor Ordinarius at the University of Kiel in 1898. In 1907, he returned to the University of Heidelberg, where he remained until his retirement in 1931.

Lenard died on 20 May 1947 in Messelhausen at the age of 84.

Lenard's major contribution to physics was in the study of cathode rays, which he began in 1888. Prior to his work, cathode rays were produced in primitive, partially evacuated glass tubes that had metallic electrodes in them, across which a high voltage could be placed. Cathode rays were difficult to study using this arrangement, because they were inside sealed glass tubes, difficult to access, and because the rays were in the presence of air molecules. He overcame these problems by devising a method of making small metallic windows in the glass that were thick enough to be able to withstand the pressure differences, but thin enough to allow passage of the rays. Having made a window for the rays, he could pass them out into the laboratory, or, alternatively, into another chamber that was completely evacuated. These windows have come to be known as Lenard windows. He was able to conveniently detect the rays and measure their intensity by means of paper sheets coated with phosphorescent and materials. In particular, he came to use pentadecylparatolylketone, which was very effective as a cathode ray detector but, unfortunately for Lenard, not fluorescent in X-rays. When Wilhelm Röntgen set out to reproduce Lenard's results, he was forced to use barium platinocyanide instead because Lenard had purchased all the available pentadecyl-para-tolyl ketone. The alternative was sensitive to both UV and X-rays allowing Röntgen to discover X-rays.

Lenard observed that the absorption of cathode rays was, to first order, proportional to the density of the material they were made to pass through. This appeared to contradict the idea that they were some sort of electromagnetic radiation. He also showed that the rays could pass through some inches of air of a normal density, and appeared to be scattered by it, implying that they must be particles that were even smaller than the molecules in air. He confirmed some of J. J. Thomson's work, which eventually arrived at the understanding that cathode rays were streams of negatively charged energetic particles. He called them quanta of electricity or for short quanta, after Helmholtz, while Thomson proposed the name "corpuscles", but eventually electrons became the everyday term. In conjunction with his and other earlier experiments on the absorption of the rays in metals, the general realization that electrons were constituent parts of the atom enabled him to claim correctly that for the most part atoms consist of empty space. He proposed that every atom consists of empty space and electrically neutral corpuscules called "dynamids", each consisting of an electron and an equal positive charge.