Pelargonium pinnatum is a stemless, tuberous perennial reaching 20 cm to 30 cm in height. The caudex may grow to 3 cm in diameter.
The hairy leaves are tufted at ground level on long stalks. They are once divided into oval segments, hence the specific name pinnatum. The leaves are mostly withered at bloomtime, which commences late in spring and continues to early autumn.
Up to eight flowers are borne per umbel, the petals obovate. They may be white, cream, pale yellow, pink, salmon or purple. Variable dark markings occur mainly on the broader upper pair of petals. In picture the funnelled lower three are marked as well. This is unusual.
The pedicel is purple in picture, the narrow sepals brown and hairy. Flower diameter is about 1,8 cm, the flower tube about 3,5 cm long.
The distribution is in the southwest of the Western Cape from the Cape Peninsula to Ceres and Albertinia.
The habitat is clay or sandstone slopes and flats. The species is not considered to be threatened in its habitat early in the twenty first century (Manning, 2007; Marais, (Ed.), 2017; Bond and Goldblatt, 1984; iNaturalist; www.bihrmann.com; http://redlist.sanbi.org).