The star cluster was observed in its infancy -- as it looked when the Universe was just three percent of its present age, NASA and the European Space Agency announced.
"We see the newly discovered galaxy, named MACS0647-JD, as it was 420 million years after the Big Bang" that created the Universe 13.7 billion years ago, a statement said.
"Its light has travelled 13.3 billion years to reach Earth."
The astronomers, grouped under the joint American-European CLASH project, use the orbiting Hubble and Spitzer telescopes as well as employing massive galaxy clusters as cosmic magnifiers to find distant galaxies.
The process, known as gravitational lensing, allows astronomers to see galaxies that emit light with a brightness weaker than that of a candle on the Moon, thus undetectable directly by telescopes on Earth.
The newly discovered cluster is so small, less than 600 light years across, that scientists believe it may still be in the first stages of galaxy formation.
Our own Milky Way is 150,000 light years across.
"The estimated mass of this baby galaxy is roughly equal to 100 million or a billion suns, or 0.1 to 1.0 percent of our Milky Way's stars," the statement said.
In September, the CLASH scientists said they had spotted the Universe's oldest and furthest galaxy, using the same technique -- the previous record-holding 13.2 billion light years away.
With gravitational lensing, theorised by Albert Einstein himself, astronomers use younger galaxies that lie closer to Earth to magnify older ones lurking in the distance by bending the light they emit.
"This latest discovery has outstripped even my expectations of what would be possible with the CLASH programme," said Rychard Bouwens of the Netherlands' Leiden University, co-author of the study to be published in the Astrophysical Journal in December.