UDF 423 is the Hubble Ultra Deep Field (UDF) identifier for a distant spiral galaxy.

With an apparent magnitude of 20,[1] UDF 423 is one of the brightest galaxies in the HUDF and also has one of the largest apparent sizes in the HUDF.

With a redshift of 1,[2] light from this galaxy is estimated to have taken around 7.7 billion years to reach Earth.

[3] However, since this galaxy is receding from Earth, the present comoving distance is estimated to be around 10 billion light-years away.

[3] In context, Hubble is observing this galaxy as it appeared when the Universe was around 4.9 billion years old.