Dysregulation of microRNAs (miRNAs) is extensively associated with cancer development and progression. miRNAs have been shown to be biomarkers for predicting tumor formation and outcome. However, identification of the relationships between miRNA expression and tumor characteristics can be difficult and time-consuming without appropriate bioinformatics expertise. To address this issue, we present OncomiR, an online resource for exploring miRNA dysregulation in cancer. Using combined miRNA-seq, RNA-seq and clinical data from The Cancer Genome Atlas, we systematically performed statistical analyses to identify dysregulated miRNAs that are associated with tumor development and progression in most major cancer types. Additional analyses further identified potential miRNA-gene target interactions in tumors. These results are stored in a backend database and presented through a web server interface. Moreover, through a backend bioinformatics pipeline, OncomiR can also perform dynamic analysis with custom miRNA selections for in-depth characterization of miRNAs in cancer.