To create a box plot in Chart.js, you can use the Chart.js plugin called Box and Violin Plot. First, you need to include the plugin in your HTML file using a script tag. Then, create a new Chart.js chart and define the type as 'boxplot'. Next, specify the data for the box plot, including the labels and datasets with the values for the boxes. Customize the appearance of the box plot by setting options such as colors, borders, and tooltips.