﻿AbstractJapanese knotweed (Reynoutriajaponica) is native to East Asia, but has been introduced to the West where it is a noxious invasive weed. Taxonomically, Japanese knotweed is placed within subtribeReynoutriinae(Polygonaceae), which also contains the austral genusMuehlenbeckia(incl.Homalocladium) and north temperateFallopia. In the current study, we conducted a phylogenetic analysis using sequence data from six markers, two nuclear (LEAFYi2, ITS) and four plastid (matK,rbcL,rps16-trnKandtrnL-trnF) to further resolve the evolutionary relationships within this group, using the widest sampling of in-group taxa to date. The results of this analysis confirmed that subtribeReynoutriinaeis a monophyletic group, characterised by the presence of extra-floral, nectariferous glands at the base of leaf petioles. Within the subtribe, four main clades were identified:Reynoutria,Fallopiasect.Parogonum,Fallopias.s. (includingFallopiasects.FallopiaandSarmentosae) andMuehlenbeckia. TheFallopias.s. andMuehlenbeckiaclades are sister to one another, while theFallopiasect.Parogonumclade is immediately basal to them andReynoutriabasal to all three.Fallopia, as currently circumscribed, is paraphyletic asMuehlenbeckiais nested within it. To resolve this, we propose that species ofFallopiasect.Parogonumshould be treated as a new genus,Parogonum(Haraldson) Desjardins & J.P.Bailey, gen. et stat. nov. WithinReynoutria, the allied specific and infraspecific taxa that fall under the name Japanese knotweed s.l. form a monophyletic group and their taxonomic status is discussed.