The Lophotrochozoa are one of two major groups of protostome animals. They include the following phyla:

• Mollusca
• Annelida
• Sipuncula
• Nemertea
• Brachiopoda
• Phoronida
• Ectoprocta
• Entoprocta

In the first four phyla there are groups that produce trochophore larvae, which have two bands of cilia around their middle. Previously these were treated together as the Trochozoa, together with the arthropods, which do not produce trochophore larvae but were considered close relatives of the annelids because they are both segmented. However, they show a number of important differences, and the arthropods are now placed separately among the Ecdysozoa.

The other four phyla are united by the presence of a lophophore, a fan of ciliated tentacles surrounding the mouth, and so were treated together as the lophophorates. They are unusual in showing radial cleavage, and some authors considered them deuterostomes, before RNA trees placed them together with the trochozoans. The exact relationships between the different phyla are not entirely certain. However, it appears that neither the lophophorates nor the Trochozoa are monophyletic groups by themselves, but are mixed together.

The flatworms and their allies form a clade, called the Platyzoa, which is closely related to the Lophotrochozoa and sometimes included within it. The name Spiralia has also been used for this extended group, since it includes all animals that develop with spiral cleavage.