Sequoioideae, commonly referred to as redwoods, is a subfamily of coniferous trees within the family Cupressaceae, that range in the northern hemisphere. It includes the largest and tallest trees in the world. The trees in the subfamily are amongst the most notable trees in the world and are common ornamental trees. The subfamily reached its peak of diversity during the early Cenozoic.

The three redwood subfamily genera are Sequoia from coastal California and Oregon, Sequoiadendron from California's Sierra Nevada, and Metasequoia in China. The redwood subfamily contains the largest and tallest trees in the world. These trees can live for thousands of years. Threats include logging, fire suppression, and burl poaching.

Only two of the genera, Sequoia and Sequoiadendron, are known for massive trees. Trees of Metasequoia, from the single living species Metasequoia glyptostroboides, are deciduous, grow much smaller (although are still large compared to most other trees) and can live in colder climates.^{[citation needed]}

Multiple studies of both morphological and molecular characters have strongly supported the assertion that the Sequoioideae are monophyletic. Most modern phylogenies place Sequoia as sister to Sequoiadendron and Metasequoia as the out-group. However, Yang et al. went on to investigate the origin of a peculiar genetic component in Sequoioideae, the polyploidy of Sequoia—and generated a notable exception that calls into question the specifics of this relative consensus.

A 2006 paper based on non-molecular evidence suggested the following relationship among extant species:

A 2021 study using molecular evidence found the same relationships among Sequoioideae species, but found Sequoioideae to be the sister group to the Athrotaxidoideae (a superfamily presently known only from Tasmania) rather than to Taxodioideae. Sequoioideae and Athrotaxidoideae are thought to have diverged from each other during the Jurassic.

Reticulate evolution refers to the origination of a taxon through the merging of ancestor lineages. Polyploidy has come to be understood as quite common in plants—with estimates ranging from 47% to 100% of flowering plants and extant ferns having derived from ancient polyploidy. Within the gymnosperms however it is quite rare. Sequoia sempervirens is hexaploid (2n= 6x= 66). To investigate the origins of this polyploidy Yang et al. used two single copy nuclear genes, LFY and NLY, to generate phylogenetic trees. Other researchers have had success with these genes in similar studies on different taxa.

Several hypotheses have been proposed to explain the origin of Sequoia's polyploidy: allopolyploidy by hybridization between Metasequoia and some probably extinct taxodiaceous plant; Metasequoia and Sequoiadendron, or ancestors of the two genera, as the parental species of Sequoia; and autohexaploidy, autoallohexaploidy, or segmental allohexaploidy.^{[citation needed]}