Powassan virus (POWV) is an emerging tick-transmitted flavivirus that circulates in North America and Russia. Up to 5% of deer ticks now test positive for POWV in certain regions of the northern United States. Although POWV infections cause life-threatening encephalitis, there is no vaccine or countermeasure available for prevention or treatment. Here, we developed a lipid nanoparticle (LNP)-encapsulated modified mRNA vaccine encoding the POWV prM and E genes and demonstrated its immunogenicity and efficacy in mice following immunization with one or two doses. The POWV mRNA vaccine induced high titers of neutralizing antibody and sterilizing immunity against lethal challenge with different POWV strains. The mRNA vaccine also induced cross-neutralizing antibodies against multiple other tick-borne flaviviruses and protected mice against the distantly related Langat virus. These data demonstrate the utility of the LNP-mRNA vaccine platform for the development of vaccines with protective activity against multiple flaviviruses. Display omitted •A Powassan virus LNP-mRNA vaccine induces potently neutralizing antibodies in mice•One dose of the mRNA vaccine protects against lethal Powassan virus challenge•The antibody response to the vaccine neutralizes other tick-borne flaviviruses•The vaccine cross-protects against disease following challenge with Langat virus VanBlargan et al. demonstrate a lipid nanoparticle-encapsulated mRNA vaccine against Powassan virus, an emerging tick-borne flavivirus, is highly immunogenic in mice and protects against lethal Powassan virus infection. Furthermore, the vaccine induces a cross-reactive antibody response against other tick-borne flavivirus that is protective against disease caused by Langat virus infection in mice.