Salvia muirii is a rigid, erect shrub reaching heights around 60 cm.
The leaves are opposite and sessile, growing in decussate pairs (perpendicular to the previous one). The leaf-shape is ovate, tapering to a bluntly pointed tip. The margins are entire, the blades finely hairy, concave on top.
The two-lipped, blue flowers have a white patch on the lower, protruding lip that is two-lobed. The upper lip is hooded, the stamens and style arched against its inside. The anthers are visible behind the far exserted style. Flowering happens from midautumn to early winter.
The species distribution is restricted around Mossel Bay and George in the southern Cape.
The plants grow on lower fynbos slopes. The species is not considered to be threatened in its habitat early in the twenty first century (Bond and Goldblatt, 1984; iNaturalist; http://redlist.sanbi.org).