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In previous works, the nearby galaxy cluster Abell 3158 was reported to possess a relatively regular, relaxed morphology in the X‐ray band. By analysing the  Chandra  and  XMM–Newton  archived data of Abell 3158, we have identified a bow‐edge‐shaped discontinuity in the X‐ray surface brightness distribution about  120  h  −1  71  kpc  west of the X‐ray peak. This feature is found to be associated with a massive, off‐centre cool gas clump, and actually forms the west boundary of the cool clump. By calculating the thermal gas pressures in the cool clump and in the free‐stream region, we determine that the cool gas clump is moving at a subsonic velocity of  700 +140  −340  km s −1  ( M = 0.6 +0.1  −0.3 )  towards west on the sky plane. We exclude the possibility that this cool clump was formed by local inhomogeneous radiative cooling in the intracluster medium, because of the effectiveness of the thermal conduction on the time‐scale of ∼0.3 Gyr. As no evidence for central active galactic nucleus activity has been found in Abell 3158, and this cool clump bears many similarities to the off‐centre cool gas clumps detected in other merging clusters in terms of their mass, size, location and thermal properties (e.g. lower temperature and higher abundance as compared with the environment), we speculate that the cool clump in Abell 3158 was caused by a merger event, and is the remnant of the original central cool‐core of the main cluster or the infalling subcluster. This idea is supported not only by the study of the line‐of‐sight velocity distribution of the cluster member galaxies, but also by the study of the gas entropy–temperature correlation. This example shows that the appearance of such massive, off‐centre cool gas clumps can be used to diagnose the dynamical state of a cluster, especially when prominent shocks and cold fronts are absent.

A joint Chandra and XMM–Newton view of Abell 3158: a massive off‐centre cool gas clump as a robust diagnostic of a merger stage