  The study of dynamically cold stellar streams reveals information about the
gravitational potential where they reside and provides important constraints on
dark matter properties. However, their intrinsic faintness makes detection
beyond Local environments highly challenging. Here we report the detection of
an extremely faint stellar stream ($\mu_{g,max}=$ 29.5 mag arcsec$^{-2}$) with
an extraordinarily coherent and thin morphology in the Coma Galaxy Cluster.
This Giant Coma Stream spans ~510 kpc in length and appears as a free-floating
structure located at a projected distance of 0.8 Mpc from the centre of Coma.
We do not identify any potential galaxy remnant or core, and the stream
structure appears featureless in our data. We interpret the Giant Coma Stream
as being a recently accreted, tidally disrupting passive dwarf. Using the
Illustris-TNG50 simulation, we identify a case with similar characteristics,
showing that, although rare, these types of streams are predicted to exist in
$\Lambda$-CDM. Our work unveils the presence of free-floating, extremely faint
and thin stellar streams in galaxy clusters, widening the environmental context
for their promising future applications in the study of dark matter properties.
