Xiyunykus pengi was named by Xu et al. (2018) [Distinguishable from other alvarezsaurians, to the extent that they are known, in possessing the following autapomorphies: large basal tubera formed exclusively by basioccipital; basisphenoid recess with basioccipital contribution and containing multiple deep fossae; cultriform process with poorly ossified dorsal margin that parallels ventral margin in lateral view; two horizontally arranged pneumatic fossae on lateral central surface of each anterior or middle cervical vertebra; distinct tubercle along ventrolateral edge of each anterior cervical centrum; groove present medial to each posterior cervical epipophysis; posterior surface of each cervical and dorsal neural arch bearing multiple deep fossae above neural canal; deep, curved groove on scapular lateral surface immediately anterior to glenoid fossa; long, deep groove along posterior edge of proximal half of scapular blade; sharp and short groove on lateral surface of femoral distal end; and proximal end of tibia with deep groove on posterior condyle.]. Its type specimen is IVPP V22783, a partial skeleton (Partial, disarticulated skeleton), and it is a 3D body fossil. Its type locality is Xiyunykus-type locality, which is in a Barremian/Aptian terrestrial horizon in the Shengjinkou Formation of China.