Ascendonanus (meaning "climbing dwarf") is an extinct genus of possible varanopid amniote from the Early Permian of Germany. It is one of the earliest specialized arboreal (tree-living) tetrapods currently known and outwardly resembled a small lizard. The animal was about 40 cm long, with strongly curved claws, short limbs, a slender, elongated trunk, and a long tail. It would have preyed on insects and other small arthropods.

The taxonomic position of varanopids is currently debated between synapsids (related to mammals, the most widely accepted idea) and diapsids (related to reptiles). The fossils of Ascendonanus are of special scientific importance because they include remains of skin, scales, scutes, bony ossicles, and soft-tissue body outlines, which could indicate that some of the oldest relatives of mammals had a scaly "reptilian-type" appearance if Ascendonanus was a synapsid.

The small varanopid Eoscansor, recently described from New Mexico, was also adapted to climbing, very likely in trees, but dates from 15 million years earlier during the Pennsylvanian subperiod of the Carboniferous, indicating that climbing amniotes have a longer history and were likely widespread.

Ascendonanus was named and described in 2018 by Frederik Spindler, Ralf Werneburg, Joerg W. Schneider, Ludwig Luthardt, Volker Annacker and Ronny Rößler from remains of five individuals that were discovered in the Chemnitz petrified forest, an Early Permian tropical fossil forest preserved under the city of Chemnitz, Germany. A Pompeii-like pyroclastic volcanic eruption 291 million years ago buried the forest and created the Zeisigwald Tuff Horizon in the uppermost Leukersdorf Formation (late Sakmarian/early Artinskian transition stage), preserving some of the animals that lived there in exceptional detail in a bottom layer of volcanic ash.

The type species is Ascendonanus nestleri. The generic name is derived from Latin ascendere, "to climb", and nanus, "dwarf". The specific name honors Knut Nestler, a long-time local supporter (deceased in 2016) of the Chemnitz Museum of Natural History (Museum für Naturkunde Chemnitz (MNC)), where the specimens of Ascendonanus are stored.

Ascendonanus was about 40 cm long, although the end of the tail is missing in all specimens and the full body length in life is not currently known. It was originally described as the smallest known member of the clade Varanopidae, a group of early synapsids that generally resembled the unrelated monitor lizards. Features that could identify Ascendonanus as a "pelycosaur" grade synapsid and a member of the Varanopidae include a single lateral temporal opening (fenestra) in the skull, a ridge on the underside of the centra of the vertebrae, and enlarged blades on the ilium of the pelvis. However, the classification of Ascendonanus as a varanopid synapsid was challenged in a recent paper that placed it with neoreptilians.

The five recovered fossils of Ascendonanus are strongly compacted and were split open as flattened counterslabs that revealed articulated partial or near complete skeletons with remains of soft tissue and some internal features. The bone material itself, however, often was not clearly preserved, making interpretation of some details more difficult. The specimens were CT scanned to reveal additional information. Based on the ossification of different bones, all individuals appear to be fully grown despite some differences in size.

The specimen designated MNC-TA0924 was made the diagnostic holotype of Ascendonanus nestleri because it provides the clearest details of the skull. The other four specimens, MNC-TA0147, MNC-TA0269, MNC-TA0906 and MNC-TA1045, were designated as the paratypes. The most remarkable specimen (MNC-TA1045) preserves the clear body outline of nearly the entire animal on counterslabs, showing the thickness in life of the limbs and the neck, and the full covering of scales.