Diverse members of the Paramyxovirus family of negative-strand RNA viruses effectively suppress host innate immune responses through the actions of their V proteins. The V protein mediates interference with the interferon regulatory RNA helicase MDA5 to avoid cellular antiviral responses. Analysis of the interaction interface revealed the MDA5 helicase C domain as necessary and sufficient for association with V proteins from human parainfluenza virus type 2, parainfluenza virus type 5, measles virus, mumps virus, Hendra virus, and Nipah virus. The identified ∼130-residue region is highly homologous between MDA5 and the related antiviral helicase LGP2, but not RIG-I. Results indicate that the paramyxovirus V proteins can also associate with LGP2. The V protein interaction was found to disrupt ATP hydrolysis mediated by both MDA5 and LGP2. These findings provide a potential mechanistic basis for V protein-mediated helicase interference and identify LGP2 as a second cellular RNA helicase targeted by paramyxovirus V proteins.