A monocyte is a white blood cell, part of the human body's immune system. It is one of the 5 major types of white blood cell, based on the appearance of white blood cells, in stained smears, as viewed under a light microscope.

Monocytes evolve from the bone marrow, circulate in the blood stream for about a day and then typically move into tissues, where, for historical reasons, they are called macrophages, or other names.

Monocytes are responsible for phagocytosis, or digestion, of foreign substances in the body. On a Wright's stained peripheral blood smear, monocytes appear larger than red blood cells and have a blueish-grey cytoplasm with a large cytoplasm to nuclear ratio. Vacuolization may be present in a cell that has recently phagocytized foreign matter.

Monocytes which migrate from the blood stream to other tissues are called macrophages. Macrophages are responsible for protecting tissues from foreign substances but are also the predominate cells involved in atherosclerosis.

A monocyte count is part of a complete blood cell count and is expressed as a ratio of monocytes to the total number of white blood cells counted.