Dicer ribonucleases are best known for their important role in microRNA and small interfering RNA biogenesis. They may also be involved in chromatin structure remodeling, apoptotic DNA degradation or production of damage-induced small RNAs, which implies that Dicer proteins can interact with many different RNA and DNA substrates. Most Dicers are multi-domain proteins. The here presented studies focus on the human Dicer (hDicer) helicase domain. The hDicer helicase contributes to recognizing pre-miRNA substrates and is suggested to participate in binding of many different cellular RNAs. However, a comprehensive characterization of the biochemical activities and substrate specificity of the hDicer helicase towards different nucleic acids have never been reported. We demonstrate for the first time that the full-length hDicer, through its helicase domain, is capable of ATP hydrolysis. We also show that the hDicer helicase binds only single- but not double-stranded nucleic acids, and that binding of single-stranded RNAs is accompanied by the rearrangement of their structure. The hDicer helicase does not require ATP hydrolysis for this RNA rearrangement activity. Given the documented importance of the hDicer helicase in antiviral defense, the obtained results may contribute to a better understanding of viral diseases and the role of hDicer in virus-host interactions.