  Galaxy clusters are the most massive gravitationally bound structures in the
Universe, comprising thousands of galaxies and pervaded by a diffuse, hot
``intracluster medium'' (ICM) that dominates the baryonic content of these
systems. The formation and evolution of the ICM across cosmic time is thought
to be driven by the continuous accretion of matter from the large-scale
filamentary surroundings and dramatic merger events with other clusters or
groups. Until now, however, direct observations of the intracluster gas have
been limited only to mature clusters in the latter three-quarters of the
history of the Universe, and we have been lacking a direct view of the hot,
thermalized cluster atmosphere at the epoch when the first massive clusters
formed. Here we report the detection (about $6\sigma$) of the thermal
Sunyaev-Zeldovich (SZ) effect in the direction of a protocluster. In fact, the
SZ signal reveals the ICM thermal energy in a way that is insensitive to
cosmological dimming, making it ideal for tracing the thermal history of cosmic
structures. This result indicates the presence of a nascent ICM within the
Spiderweb protocluster at redshift $z=2.156$, around 10 billion years ago. The
amplitude and morphology of the detected signal show that the SZ effect from
the protocluster is lower than expected from dynamical considerations and
comparable with that of lower-redshift group-scale systems, consistent with
expectations for a dynamically active progenitor of a local galaxy cluster.
