Previous phylogenetic analyses indicated that Polygonoideae, the largest subfamily in the Polygonaceae, is monophyletic. Phylogenetic relationships within the Polygonoideae have been substantially controversial. We collected 160 samples representing all currently recognized tribes for a more comprehensive phylogenetic analysis of the subfamily. Here, we reconstructed phylogenetic relationships of the Polygonoideae, inferred ancestral character states, and estimated the divergence time with a dense taxon sampling. This study corroborated and expanded previous results regarding the phylogenetic relationships of the Polygonoideae clade, and resolved the phylogenetic status of some controversial taxa by integrating molecular and morphological evidence. Phylogenetic analyses based on the complete plastomes suggested strong support for six primary clades that correspond to the most recent circumscription of tribes: Polygoneae, Rumiceae, Calligoneae, Pteroxygoneae, Fagopyreae, and Persicarieae. In addition, we provided further morphological data and assessed characters that supported different clades. The 3‐colpate pollen, 5‐parted perianth, and 3 styles were inferred to be the ancestral states of Polygonoideae. Divergence time estimation revealed that Polygonoideae originated around the late Cretaceous, and diversification was concentrated in the Eocene and Miocene. Time estimation indicated that the rapid uplift of the Tibetan Plateau and the intensification of the Asian monsoons might be potential driving forces for the diversification of Polygonoideae. Overall, this study advances our understanding of the phylogeny and diversification of the Polygonoideae and highlights the adaptive evolution of the taxa.