Petrophyton caespitosum (orth. var. Petrophytum caespitosum) is a species of flowering plant in the rose family known by the common name mat rock spiraea. It is native to the western United States where it grows in mountainous areas from the Sierra Nevada to the Rocky Mountains. It grows among rocks in forested and woodland habitat. This is a very low matted shrub growing in carpets up to 80 centimeters wide, creeping over rocks. The plant often grows on vertical surfaces and hangs by its roots, which cling to cracks in rock. The stems are thick and very short, covered densely in rosettes of oval leaves. It produces many flower clusters which are spikelike clusters of flowers arising on erect peduncles up to 10 centimeters tall. Each flower cluster is packed with flowers, each with five tiny white petals, many whiskery stamens, and usually 5 hairy pistils. The flower parts dry and may fall away, leaving a cluster of developing fruits, follicles containing the seeds.