The eukaryotic transcriptome contains numerous genes that express multiple RNA products (RNA isoforms) through processes such as alternative splicing. RNA isoforms have different exonic content and can perform different biological functions. The scale of alternative isoforms is huge (the current GENCODE 47 release contains 78,724 genes and 385,659 isoforms in humans), and novel isoforms continue to be discovered. To comprehend the growing number of RNA isoforms, a variety of bioinformatics software has been developed for isoform visualization and analysis. However, they suffer from usability and functionality limitations, such as requiring programming knowledge, being unable to visualize user data and isoform expression levels, or not illustrating how changes in exonic structures affect the encoded protein. To overcome these limitations, we developed the Isoform Visualizer (IsoVis1), a user-friendly website that accepts user-supplied transcriptomic data and intuitively visualizes both isoform structures and expression levels. It is built with the Nuxt 2 framework and written in Vue.js and BootstrapVue. IsoVis accepts a variety of input files (GTF, GFF, BED) to create an 'isoform stack' visualization, where each row corresponds to one isoform. An optional isoform expression data file (CSV, tab-separated text) can also be provided, with expression levels presented as a heatmap beside the stack. IsoVis has many features, including broad species compatibility; integration of Ensembl gene and isoform annotations; visualization of open reading frames in isoforms; incorporation of InterPro annotations to show the mapping of protein domains and motifs to exons; and high quality image export. All data visualized on IsoVis remain private to the user, making it suitable for analyzing sensitive data. IsoVis provides an intuitive view of how gene isoforms differ and diverge, allowing researchers to explore the rich isoform-level data produced by RNA-seq and gain new insights into alternative isoform expression and functions.