Most baryons in present-day galaxy clusters exist as hot gas (≳107 K), forming the intracluster medium (ICM)1. Cosmological simulations predict that the mass and temperature of the ICM decline towards earlier times, as intracluster gas in younger clusters is still assembling and being heated2–4. To date, hot ICM has been securely detected only in a few systems at or above z ≈ 2, leaving the timing and mechanism of ICM assembly uncertain5–7. Here we report the direct observation of hot intracluster gas via its thermal Sunyaev–Zeldovich signature in the protocluster SPT2349–56 with the Atacama Large Millimeter/submillimeter Array. SPT2349–56 hosts a large molecular gas reservoir and three radio-loud active galactic nuclei (AGN) within an approximately 100-kpc region at z = 4.3 (refs. 8–11). The measurement implies a thermal energy of about 1061 erg in the core, about 10 times more than gravity alone should produce. Contrary to current theoretical expectations3,4,12, the hot ICM in SPT2349–56 demonstrates that substantial heating can occur very early in cluster assembly, depositing enough energy to overheat the nascent ICM well before mature clusters become common at z ≈ 2.
