Magnoliids, Magnoliidae or Magnolianae are a clade of flowering plants. With more than 10,000 species, including magnolias, nutmeg, bay laurel, cinnamon, avocado, black pepper, tulip tree and many others, it is the third-largest group of angiosperms after the eudicots and monocots. The group is characterized by trimerous flowers, pollen with one pore, and usually branching-veined leaves.

Some members of the subclass are among the earliest angiosperms and share anatomical similarities with gymnosperms like stamens that resemble the male cone scales of conifers and carpels found on the long flowering axis. According to molecular clock calculations, the lineage that led to magnoliids split from other plants about 135 million years ago or 160-165 million years ago.

"Magnoliidae" is the botanical name of a subclass, and "magnoliids" is an informal name that does not conform to the International Code of Nomenclature for algae, fungi, and plants. The circumscription of a subclass will vary with the taxonomic system being used. The only requirement is that it must include the family Magnoliaceae. The informal name "magnoliids" is used by some researchers to avoid the confusion that recently surrounds the name "Magnoliidae." More recently,^{[clarification needed]} the group^{[clarification needed]} has been redefined under the PhyloCode as a node-based clade comprising the Canellales, Laurales, Magnoliales, and Piperales.^{[citation needed]} Chase & Reveal have proposed "Magnoliidae" as the name used for the entire group of flowering plants, and the formal name "Magnolianae" for the group of four orders discussed here.

The APG III (2009) and its predecessor systems did not originally use formal botanical names above the rank of order. Under those systems, larger clades were usually referred to by informal names, such as "magnoliids" (plural, not capitalized) or "magnoliid complex". The formal name in Linnean nomenclature was specified in a separate APG publication as the existing name "Magnolianae" Takht. (1967). The APG III recognizes a clade within the angiosperms for the magnoliids. The circumscription is:

The clade includes most of the basal groups of the angiosperms. This clade was formally named Magnoliidae in 2007 under provisions of the PhyloCode.

The Cronquist system (1981) used the name Magnoliidae for one of six subclasses (within class Magnoliopsida = dicotyledons). In the original version of this system the circumscription was:

Both Dahlgren and Thorne classified the magnoliids (sensu APG) in superorder Magnolianae, rather than as a subclass. In their systems, the name Magnoliidae is used for a much larger group including all dicotyledons. This is also the case in some of the systems derived from the Cronquist system.^{[citation needed]}

Dahlgren divided his Magnolianae into ten orders, more than other systems of the time, and unlike Cronquist and Thorne, he did not include the Piperales. Thorne grouped most of his Magnolianae into two large orders, Magnoliales and Berberidales, although his Magnoliales was divided into suborders along lines similar to the ordinal groupings used by both Cronquist and Dahlgren. Thorne revised his system in 2000, restricting the name Magnoliidae to include only the Magnolianae, Nymphaeanae, and Rafflesianae, and removing the Berberidales and other previously included groups to his subclass Ranunculidae. This revised system diverges from the Cronquist system, but agrees more closely with the circumscription later published under APG II.^{[citation needed]}