In the last decade, a powerful biotechnological tool for the in vivo and in vitro specific labeling of proteins (SNAP-tag(™) technology) was proposed as a valid alternative to classical protein-tags (green fluorescent proteins, GFPs). This was made possible by the discovery of the irreversible reaction of the human alkylguanine-DNA-alkyl-transferase (hAGT) in the presence of benzyl-guanine derivatives. However, the mild reaction conditions and the general instability of the mesophilic SNAP-tag(™) make this new approach not fully applicable to (hyper-)thermophilic and, in general, extremophilic organisms. Here, we introduce an engineered variant of the thermostable alkylguanine-DNA-alkyl-transferase from the Archaea Sulfolobus solfataricus (SsOGT-H(5)), which displays a catalytic efficiency comparable to the SNAP-tag(™) protein, but showing high intrinsic stability typical of proteins from this organism. The successful heterologous expression obtained in a thermophilic model organism makes SsOGT-H(5) a valid candidate as protein-tag for organisms living in extreme environments.