Summary:Highly multiplexed imaging enables single-cell-resolved detection of numerous biological molecules in their spatial tissue context. Interactive data visualization of multiplexed imaging data is necessary for quality control and hypothesis examination. Here, we describecytoviewer, an R/Bioconductor package for interactive visualization and exploration of multi-channel images and segmentation masks. Thecytoviewerpackage supports flexible generation of image composites, allows side-by-side visualization of single channels, and facilitates the spatial visualization of single-cell data in the form of segmentation masks. The package operates onSingleCellExperiment, SpatialExperimentandCytoImageListobjects and therefore integrates with the Bioconductor framework for single-cell and image analysis. Users ofcytoviewerneed little coding expertise, and the graphical user interface allows user-friendly navigation. We showcase the functionality ofcytoviewerby analysis of an imaging mass cytometry dataset of cancer patients.