Transglutaminase 2 (TG2) is the first described cellular member of an enzyme family catalyzing Ca²⁺‐dependent transamidation of proteins. During the last two decades its additional enzymatic (GTP binding and hydrolysis, protein disulfide isomerase, protein kinase) and non‐enzymatic (multiple interactions in protein scaffolds) activities, which do not require Ca²⁺, have been recognized. It became a prevailing view that TG2 is silent as a transamidase, except in extreme stress conditions, in the intracellular environment characterized by low Ca²⁺ and high GTP concentrations. To counter this presumption a critical review of the experimental evidence supporting the role of this enzymatic activity in cellular processes is provided. It includes the structural basis of TG2 regulation through non‐canonical Ca²⁺ binding sites, mechanisms making it sensitive to low Ca²⁺ concentrations, techniques developed for the detection of protein transamidation in cells and examples of basic cellular phenomena as well as pathological conditions influenced by this irreversible post‐translational protein modification.