Alan Harvey Guth is an American theoretical physicist and cosmologist. Guth has researched elementary particle theory (and how particle theory is applicable to the early universe). He is currently serving as Victor Weisskopf Professor of Physics at the Massachusetts Institute of Technology, and he is the originator of the inflationary universe theory.

He graduated from MIT with a degree in physics and stayed to receive a Master's and a doctorate, also in physics. Guth has worked at Princeton, Columbia, Cornell, and the Stanford Linear Accelerator Center (SLAC) .

At the start of his career, Guth studied particle physics, not cosmology. Guth's earliest work at Princeton was in the study of quarks, the elementary particles that make up protons and neutrons. At Columbia, Guth studied grand unification (GUTs), focusing on the phase transitions generated by spontaneous symmetry breaking. Most GUTs predict the generation of magnetic monopoles during spontaneous symmetry breaking, but none had ever been detected - the monopole problem.

Guth is the Victor F. Weisskopf Professor of Physics at the Massachusetts Institute of Technology (MIT). So far, he has written about 60 technical papers related to the effects of inflation and its interactions with particle physics. He has won many awards and medals, including the Medal of the International Center for Theoretical Physics, Trieste, Italy, with Andrei Linde and Paul Steinhardt and the Eddington Medal in 1996, and the 2009 Isaac Newton Medal, awarded by the British Institute of Physics.

In July 2012, he was an inaugural awardee of the Fundamental Physics Prize, the creation of physicist and internet entrepreneur, Yuri Milner.