Synovial membrane (or synovium) is the soft tissue found between the articular capsule (joint capsule) and the joint cavity of synovial joints.

The word "synovium" is related to the word "synovia" (synovial fluid), which is the clear, viscid, lubricating fluid secreted by synovial membranes. The word "synvovia" or "sinovia" was coined by Paracelsus, and may have been derived from the Greek word "syn" ("with") and the Latin word "ovum" ("egg") because the synovial fluid in joints that have a cavity between the bearing surfaces is similar to egg white.

Synovium is very variable but often has two layers.

Where the underlying subintima is loose, the intima sits on a pliable membrane, giving rise to the term synovial membrane.

This membrane, together with the cells of the intima, provides something like an inner tube, sealing the synovial fluid from the surrounding tissue (effectively stopping the joints from being squeezed dry when subject to impact, such as running).

The intimal cells are of two types, fibroblasts and macrophages, both of which are different in certain respects from similar cells in other tissues.