A sinusoid is a small blood vessel similar to a capillary but with a discontinuous endothelium.

Sinusoids are found in the liver, lymphoid tissue, endocrine organs, and hematopoietic organs such as the bone marrow and the spleen. Sinusoids found within terminal villi of the placenta are not comparable to these; they possess a continuous endothelium and complete basal lamina.

Sinusoids are highly permeable, having larger inter-cellular clefts, fewer tight junctions, and discontinous endothelial cells. The level of permeability is such as to allow small and medium-sized proteins such as albumin to enter and leave the blood stream. Some spaces are large enough for blood cells to pass^{citation needed}.

The sinusoids of the liver are of particular importance to the function of that organ, and are discussed in more detail at liver sinusoid.

Fenestrations

In sinusoid blood vessels, fenestrations are gaps in between the endothelial cells, increasing permeability.

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