Soil profiles and archaeological sites in Northeast Iceland contain a sequence of basaltic tephra layers coinciding in time with the first human settlement of the area during the Viking Age, known as the Landnám tephra sequence. The chronology of these layers is useful when reconstructing the history of human settlement in Iceland and its environmental impact. The only properly dated tephra layer from this tephra sequence is the Landnám tephra layer (LTL), formed in the A.D. 870s. Sedimentation rates calculated from the interval between tephra layers of known age (A.D. 871 ± 2, 1158, and 1300) in high-resolution sediment cores from Lake Mývatn were used to establish the approximate age of six tephra layers in the medieval period. One of the main objectives of the study was to improve dating of a younger tephra layer, formed in the mid-10^th century according to previous studies. This tephra layer, originating from the Veiðivötn volcanic system, has proven to be a very important marker bed for archaeological research in the Mývatn area. The results indicate that the 10^th-century Veiðivötn tephra formed in the period A.D. 930–940. The paper proposes the name V-Sv for this tephra layer. Tephrochronology established from lacustrine sediment cores with high sedimentation rates can provide valuable additional information for constructing chronologies at archaeological sites in the North Atlantic.