A curved belt consisting of terranes or blocks and associated late Paleozoic to early Mesozoic sutures are features of the southeastern part of the Tibetan Plateau. The southeastern Tibetan Plateau Paleotethyan belt is divided into a northwestern segment (central Tibetan Plateau) and a southern segment (the Sanjiang orogenic belt). The Paleotethyan tectonic history of this belt remains a subject of considerable debate. A major controversy concerns whether or not it evolved as a single genetic element. This paper provides new geochronological, geochemical, and structural data to constrain the Paleotethyan tectonics of this belt. A detailed field study across three sections of the Sanjiang orogenic belt has identified two volcaniclastic successions separated by a regional angular unconformity in the northwestern segment, and one continuous volcaniclastic succession in the southern segment. New zircon U-Pb ages reveal that: (1) the lower succession of the northwestern segment was produced by three volcanic phases at ca. 272 Ma, 253–247 Ma, and 240–235 Ma; (2) the upper succession of the northwestern segment formed during a shorter period along the Yushu–Yidun arc from ca. 227 to 211 Ma; and (3) the single volcaniclastic succession of the southern segment was produced continuously from ca. 260 to 210 Ma. Whole-rock geochemical data including trace elements show that the late Paleozoic to early Mesozoic magmatic activity has a continental arc–like affinity. The spatial distribution and field relationships of the magmatic rocks define two continuous volcanic arcs in the southeastern Tibetan Plateau, which correspond to two phases of arc-like magmatism. The Late Triassic Yushu–Yidun arc belt is exposed along the northern half of the northwestern segment to the south of the western Jinsha–Garzê–Litang suture. Southward subduction along the western Jinsha–Garzê–Litang suture is likely to have generated the Yushu–Yidun arc. The second arc (Jomda–Weixi–Yunxian) was diachronous and is continuous through the southeastern Tibetan Plateau. The northwestern part of the Jomda–Weixi–Yunxian arc belt is overlain by the Yushu–Yidun arc from the north. From northwest to southeast, the cessation of magmatism in the Jomda–Weixi–Yunxian arc varies from the Middle Triassic (ca. 235 Ma) to the Late Triassic (ca. 210 Ma), whereas the onset of magmatism appears to have been in the Early Permian throughout the arc. Zircon Lu-Hf isotopic data suggest that large degrees of melting of a depleted mantle source produced the earliest magmas in each segment of the Jomda–Weixi–Yunxian arc. The similarity in zircon ages of the ophiolitic units and their distribution imply that the Longmu Co–Shuanghu suture is a continuation of the Changning–Menglian suture. The spatial relationship between the Jomda–Weixi–Yunxian arc and the Longmu Co–Shuanghu–Changning–Menglian suture suggests that diachronous eastward subduction of part of the Paleotethys along the suture produced the Jomda–Weixi–Yunxian arc belt. Widespread folds of the continental arc–volcaniclastics with orogen-parallel hinges reflect compression perpendicular to the orogen, subsequent to arc magmatism. As such, the Paleotethyan tectonics of the southeastern Tibetan Plateau are characterized by two-phase subduction and subsequent two-phase continent-continent collision.