The potentially habitable planet, named "TOI 700 d," is relatively close to Earth - only 100 light years away, NASA's Jet Propulsion Laboratory announced during the winter American Astronomical Society meeting in Honolulu, Hawaii on Monday.
Proxima Centauri, the closest star to Earth, is just over four light years away.
"TESS was designed and launched specifically to find Earth-sized planets orbiting nearby stars," said Paul Hertz, NASA astrophysics division director.
TESS initially misclassified the star, which meant the planets appeared larger and hotter than they actually are. But several amateur astronomers, including high school student Alton Spencer - who works with members of the TESS team - identified the error.
"When we corrected the star's parameters, the sizes of its planets dropped, and we realized the outermost one was about the size of Earth and in the habitable zone," said Emily Gilbert, a graduate student at the University of Chicago.
The discovery was later confirmed by the Spitzer Space Telescope.
A few other similar planets have been discovered before, notably by the Kepler Space Telescope, but this is the first discovered by TESS, which was launched in 2018.
TESS stabilizes on one area of the sky to detect whether objects - planets - pass in front of stars, which causes a temporary drop in the stars' luminosity. This allows TESS to infer the presence of a planet, its size and orbit.
Star TOI 700 is small, about 40 percent of our Sun's size and only about half as hot.
TESS discovered three planets in orbit, named TOI 700 b, c and d. Only "d" is in the so-called "Goldilocks zone," not too far from and not too close to the star, where the temperature could allow the presence of liquid water.
The name is an allusion to the fairy tale "Goldilocks and the Three Bears," in which a young girl samples three bowls of porridge and finds that one is "just right - not too hot and not too cold." TOI 700 d is about 20 percent larger than Earth and orbits its star in 37 days. It receives 86 percent of the energy that Earth receives from the Sun.