iheatmapr is an R package for building complex, interactive heatmaps using modular building blocks. "Complex" heatmaps are heatmaps in which subplots along the rows or columns of the main heatmap add more information about each row or column. For example, a one column additional heatmap may indicate what group a particular row or column belongs to. Complex heatmaps may also include multiple side by side heatmaps which show different types of data for the same conditions. Interactivity can improve complex heatmaps by providing tooltips with information about each cell and enabling zooming into interesting features. iheatmapr uses the plotly library for interactivity.While there are already plenty of awesome R packages for making heatmaps, including several great packages for making relatively simple interactive heatmaps (heatmaply and d3heatmap) or complex static heatmaps (ComplexHeatmap), iheatmapr seeks to make it easy to make complex interactive heatmaps.