Fjällsjöälven is a river in northwestern Ångermanland, Strömsund municipality.
The Fjällsjöälven has its headwaters in the border region between Lapland and Jämtland with a northern source, the Saxån, and a southern source, the Sjoutälven, which unite in the four-mile-long Tåsjön (248 m above sea level).
Below Tåsjön, Fjällsjöälven also receives water from Flåsjöån, Rörströmsälven and Hotingsån, when they merge in Bodumsjön (212 m above sea level) at Rossön (Bodums parish) in Strömsund municipality.
Fjällsjöälven flows into Ångermanälven about 5 km northwest of Näsåker.
The largest tributary of the Fjällsjöälven is the Vängelälven, which transfers water from the Faxälven.