Polypsecadium is a genus of large herbaceous species of plants in the family Brassicaceae, found growing in South America.

[2][5] The genus remained monotypic until 1982, when the Argentine botanists M. C. Romanczuk and Osvaldo Boelcke added two new species.

[6] In their 2003 general work on Brassicaceae (Cruciferae) in the book series The families and genera of vascular plants, O. Appel and the Iraqi botanist Ihsan Ali Al-Shehbaz continued to recognise three species in the genus, but by this time, in the beginning of the 2000s, phylogenetic trees produced by molecular studies of the DNA were challenging the circumscription of the traditional genera -in other words, the traditional morphological characteristics were proving unreliable in delimiting monophyletic genera.

One of these genera was Sisymbrium, which consisted of some 90 species from the end of the 1980s to the early 2000s, a large proportion of these from South America.

[7] The North American species of Sisymbrium had already been moved to new genera in 1976 by Rollins, except one (the wrong one, it later turned out).

[8] In the dichotomous key provided by Al-Shehbaz in 2006, Polypsecadium is distinguished from the other South American Schizopetaleae by virtue of its seeds being biseriate (in two rows in the silique) or sub-biseriate (in the case of P. brasiliense), the stigma is retained in the mature fruit and is much wider than style (except in P. litorale and P. llatasii), and the cauline leaves have longish petioles of some 1-7cm long.

The inflorescence is an ebracteate, corymbose, lax (not stiffly erect) raceme with many flowers which elongates considerably when in fruit.

The fruit of Brassicaceae are known as 'siliques', in this genus they are dehiscent (splitting open when dry), non-curved, linear, terete and unsegmented.

P. adscendens, from high elevations in the Andes mountains of Boyacá in Colombia and Pichincha in Ecuador, is perhaps the most odd: it is poorly known, having only been collected a handful of times since its discovery approximately a century ago, but it appears to be a cruciferous vine, scrambling up trees to some five to nine meters from the ground.

[10] As of 2017, the fifteen species accepted in the Plants of the World Online database,[2] as well as the BrassiBase project of the Heidelberg University as of 2010,[10] are: