MicroRNAs (miRNAs) are a class of small, single-stranded non-coding RNAs that act as important regulators of gene expression and are involved in a number of important processes in life. A large number of studies have suggested that dysregulation of miRNA expression may be an important part of the mechanism of human tumorigenesis and progression. MiR-155-5p is mainly regarded as an oncomiR that acts on multiple target genes to participate in tumor progression, although it has been suggested to possess cancer growth suppressor effects. In this paper, we summarize the effects of miR-155-5p on cancer cell proliferation, invasion, migration, and drug resistance in various tumor types and elucidate its value as a possible potential marker in assisting diagnosis.