The molecular phylogeny of brown algae was examined using concatenatedDNAsequences of seven chloroplast and mitochondrial genes (atpB,psaA,psaB,psbA,psbC,rbcL, andcox1). The study was carried out mostly from unialgal cultures; we includedPhaeostrophion irregulareandPlatysiphon glacialisbecause their ordinal taxonomic positions were unclear. Overall, the molecular phylogeny agreed with previously published studies, however,Platysiphonclustered withHalosiphonandStschapoviaand was paraphyletic with the Tilopteridales.PlatysiphonresembledStschapoviain showing remarkable morphological changes between young and mature thalli.Platysiphon,HalosiphonandStschapoviaalso shared parenchymatous, terete, erect thalli with assimilatory filaments in whorls or on the distal end. Based on these results, we proposed a new order Stschapoviales and a new family Platysiphonaceae. We proposed to includePhaeostrophionin the Sphacelariales, and we emended the order to include this foliose member. Finally, using basal taxa not included in earlier studies, the origin and divergence times for brown algae were re‐investigated. Results showed that the Phaeophyceae branched from Schizocladiophyceae ~260 Ma during the Permian Period. The early diverging brown algae had isomorphic life histories, whereas the derived taxa with heteromorphic life histories evolved 155–110 Ma when they branched from the basal taxa. Based on these results, we propose that the development of heteromorphic life histories and their success in the temperate and cold‐water regions was induced by the development of the remarkable seasonality caused by the breakup of Pangaea. Most brown algal orders had diverged by roughly 60 Ma, around the last mass extinction event during the Cretaceous Period, and therefore a drastic climate change might have triggered the divergence of brown algae.