||Pachypleurosaurs (Pachypleurosauroidea) are a group of small to medium-sized, lizard-like marine reptiles in the Early to Middle Triassic, including Pachypleurosauridae, Keichousauridae and closely related taxa. The group is generally considered as a sauropterygian radiation, but its phylogenetic interrelationships remain highly debated. Here, we present a new pachypleurosaurid, Honghesaurus longicaudalis gen. et sp. nov., from the early Middle Triassic (Anisian,？~？244 Ma) marine deposits in Luxi, Yunnan, China. The discovery documents the first really long-tailed pachypleurosaur with totally 121 (69 caudal) vertebrae, providing new evidence for the vertebral multiplication and ecological adaption of this group. The long trunk associated with an incredibly long tail could provide Honghesaurus the advantage of maneuverability and energy efficiency for lateral undulatory swimming. Honghesaurus, although possessing a series of autapomorphies, fills the morphological gap between Qianxisaurus from the Ladinian Xingyi Biota and Wumengosaurus from the Anisian Panxian Biota. Phylogenetic studies unite these three pachypleurosaurids as a monophyletic clade above European pachypleurosaurid clades and provide new insights into the interrelationships of this group. Our scenario of pachypleurosaurian phylogeny combined with the stratigraphic data imply that the Tethys Ocean was a west–east corridor for dispersal of pachypleurosaurids from Europe into South China.