Alexei Vladimir "Alex" Filippenko (/fɪlɪˈpɛnkoʊ/; born July 25, 1958) is an American astrophysicist and professor of astronomy at the University of California, Berkeley. Filippenko graduated from Dos Pueblos High School in Goleta, California. He received a Bachelor of Arts in physics from the University of California, Santa Barbara in 1979 and a Ph.D. in astronomy from the California Institute of Technology in 1984, where he was a Hertz Foundation Fellow. He was a postdoctoral Miller Fellow at Berkeley from 1984 to 1986 and was appointed to Berkeley's faculty in 1986. In 1996 and 2005, he was a Miller Research Professor, and he is currently a Senior Miller Fellow. His research focuses on supernovae and active galaxies at optical, ultraviolet, and near-infrared wavelengths, as well as on black holes, gamma-ray bursts, and the expansion of the Universe.

Filippenko is the only person who was a member of both the Supernova Cosmology Project and the High-z Supernova Search Team, which used observations of extragalactic Type Ia supernovae to discover the accelerating universe and its implied existence of dark energy. The discovery was voted the top science breakthrough of 1998 by Science magazine and resulted in the 2011 Nobel Prize in Physics being awarded to the leaders of the two project teams.

Filippenko developed and runs the Katzman Automatic Imaging Telescope (KAIT), a fully robotic telescope which conducts the Lick Observatory Supernova Search (LOSS). During the years 1998–2008, it was by far the world's most successful search for relatively nearby supernovae, finding over 650 of them.

His research, concentrating on optical spectroscopy, showed that many core-collapse supernovae result from massive stars with partially or highly stripped envelopes, helped establish the Type IIn subclass characterized by ejecta interacting with circumstellar gas, observationally identified the progenitors of some supernovae, revealed that many supernovae are quite aspherical, and showed that Type Ia supernovae exhibit considerable heterogeneity—crucial to the development of methods to calibrate them for accurate distance determinations.

Filippenko's early work showed that the nuclei of most bright, nearby galaxies exhibit activity physically similar to that of quasars, driven by gas accretion onto a supermassive black hole. He is also a member of the Nuker Team which uses the Hubble Space Telescope to examine supermassive black holes and determined the relationship between a galaxy's central black hole's mass and velocity dispersion. In half a dozen X-ray binary stars, he provided compelling dynamical evidence for a stellar-mass black hole. His robotic telescope (KAIT) made some of the very earliest measurements of the optical afterglows of gamma-ray bursts.

The Thompson-Reuters "incites" index ranked Filippenko as the most cited researcher in space science for the ten-year period between 1996 and 2006.

Filippenko is frequently featured in the History Channel series The Universe, as well as in the series How the Universe Works. Overall, he has participated in more than 120 science documentaries.

Filippenko is the author of and teacher in an eight-volume teaching series on DVD called Understanding the Universe. Organized into three major sections in ten smaller units, this series of 96 half-hour lectures covers the material of an undergraduate survey course for An Introduction to Astronomy (the series' subtitle). His other videos courses are Black Holes Explained and Skywatching: Seeing and Understanding Cosmic Wonders.