Malacothamnus arcuatus is a species of flowering plant in the mallow family, which has two varieties, one of which is sometimes recognized as the species Malacothamnus hallii.

The two varieties are mostly morphologically distinct but intergrade where their geographic ranges meet.

These varieties together form a single clade in phylogenetic analyses with some analyses providing evidence supporting two taxa.

When treated at the species rank, it is given the name M. hallii.

It is endemic to the southern Coastal Ranges of California, primarily in the northern half of the Diablo Range and areas in-between it and the Santa Cruz Mountains.