Conjugative plasmids are self-transmissible mobile genetic elements that transfer DNA between host cells via type IV secretion systems (T4SS). While T4SS-mediated conjugation has been well-studied in bacteria, information is sparse in Archaea and known representatives exist only in the Sulfolobales order of Crenarchaeota. Here we present the first self-transmissible plasmid identified in a Euryarchaeon, Thermococcus sp. 33-3. The 103 kbp plasmid, pT33-3, is seen in CRISPR spacers throughout the Thermococcales order. We demonstrate that pT33-3 is a bona fide conjugative plasmid that requires cell-to-cell contact and is dependent on canonical, plasmid-encoded T4SS-like genes. Under laboratory conditions, pT33-3 transfers to various Thermococcales and transconjugants propagate at 100 °C. Using pT33-3, we developed a genetic toolkit that allows modification of phylogenetically diverse Archaeal genomes. We demonstrate pT33-3-mediated plasmid mobilization and subsequent targeted genome modification in p re vi ously u nt ransformable Thermococcales species, and extend this process to interphylum transfer to a Crenarchaeon.