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During the past two decades, O‐GlcNAc modification of cytosolic and nuclear proteins has been intensively studied. Nevertheless, the function of this post‐translational modification remains unclear. It has been recently speculated that O‐GlcNAc could act as a protective signal against proteasomal degradation, both by modifying target substrates and/or by inhibiting the proteasome itself. In this work, we have investigated the putative relation between O‐GlcNAc and the ubiquitin pathway. First, we showed that the level of both modifications increased rapidly after thermal stress but, unlike ubiquitinated proteins, O‐GlcNAc‐modified proteins failed to be stabilized by inhibiting proteasome function. Increasing O‐GlcNAc levels, using glucosamine or PUGNAc, enhanced ubiquitination. Inversely, when O‐GlcNAc levels were reduced, using forskolin or glucose deprivation, ubiquitination decreased. Targeted‐RNA …