Walter Hendrik Gustav Lewin (born January 29, 1936) is a Dutch astrophysicist and retired professor of physics at the Massachusetts Institute of Technology.

Lewin's contributions in astrophysics include the first discovery of a rotating neutron star through all-sky balloon surveys and research in X-ray detection in investigations through satellites and observatories.

Lewin has received awards for teaching and is known for his lectures on physics and their publication online via YouTube, MIT OpenCourseWare and edX.

[5][6] To protect the family, Lewin’s father — who was Jewish, unlike his mother — decided one day to simply leave without telling anyone.

After the war ended, his father resurfaced; Lewin describes having a “more or less normal childhood.” His parents continued running the school, which he says strongly influenced his love of teaching.

[4][7] Walter Lewin taught high school physics while studying for his PhD, then he went to Massachusetts Institute of Technology in January 1966 as a post-doctoral associate, and was appointed an assistant professor.

The rockets used by other researchers could not have discovered that the X-ray sources varied on such short time scales because they were only up for several minutes, whereas the balloons could be in the air for many hours.

Lewin and his graduate student Eugene Magnier have made deep optical charge-coupled device observations of M31 in four colors; they have published a catalogue of 500,000 objects.

Lewin and his graduate student David Pooley initiated the successful X-ray observations within six days of the appearance of supernova SN 1993J in M81.

In 1996–1998, Lewin's collaboration with Michiel van der Klis in Amsterdam led to the discovery of kHz oscillations in many X-ray binaries.

Evidence was found for spectral distortions of the iron line (in X-rays) indicative of the influence of general relativity on the iron-line emission in the vicinity of the "event horizon" of the black holes.

[27] In the summer of 2012, Lewin returned from his retirement to deliver a lecture series initiated and funded by the Japanese Broadcasting Corporation (NHK).

[1] Inside Higher Ed reported that this learner was one of at least 10 female students to whom Lewin had sent inappropriate messages, which MIT confirmed.

[30] The victim, a 32-year-old woman living in France, said that she came forward to ensure that the case was not forgotten, stating that Lewin pushed her to participate in sexual role-playing and send naked pictures of herself.

[30] As a consequence of its internal investigation, MIT revoked Lewin's professor emeritus title,[2] and removed his lectures from the institute's online learning platforms.