3D bioprinting utilizes a cell-laden bioink to fabricate 3D cellular constructs for a variety of biomedical applications. The printing process typically takes hours to fabricate heterogeneous artificial tissues with multiple types of cells, different types of extracellular matrices, and interconnected vascular networks. During the printing process, the suspended cells sediment within the bioink with time, resulting in inhomogeneous cell concentration, which significantly affects the printing reliability and accuracy. This paper is the first study to quantify the cell sedimentation process in the bioink containing living cells. In this study, the effects of polymer concentration and standing time on the cell sedimentation velocity and cell concentration have been systematically investigated. The main conclusions are (1) the cell sedimentation velocity is almost constant at different standing times, because the cell gravitational force is balanced by the cell buoyant force and the drag force; (2) with the increase of the polymer concentration, the cell sedimentation velocity decreases, while the cell mass density increases due to less water absorbed; (3) with the increase of the standing time, the cell concentration near the bottom of the bioink reservoir increases linearly. With the increase of the polymer concentration, this linear increase of the cell concentration with the standing time significantly slows down due to a significant decrease of the cell sedimentation velocity; and (4) for the bioink with a low sodium alginate concentration, cell concentration near the bottom of the bioink reservoir is not uniform, and cell aggregates are observed.