It is for the first time that rocks of the Kostomuksha Greenstone Belt have been found to contain zircons of Mesoproterozoic (1545.9 ± 3.5 Ma) age. Zircons to be dated were separated from the southern portion of a metarhyolite (halleflinta) sill occurring among the schists and banded iron formation in the Kostomuksha suite of the Gimoly series. The zircons occur as transparent, pale-pink elongated and lesser short-prismatic crystals varying in size from 60 to 400 μm. The zircons were dated by two methods: 1) laser ablation (LA-ICP-MS) and 2) isotopic dilution U-Pb CA-ID-TIMS). The former method yielded ages of 1538 ± 9 and 1535 ± 9 Ma and the latter 1546 ± 4 Ma. The results were comparable within the error limits. Dating by laser ablation revealed Mesoarchean (2949 ± 7, 2885 ± 8 and 2830 ± 8 Ma) grains among the zircons. Laser ablation was used to analyze the geochemical features of Mesoproterozoic zircons. They are poorer in light REE than Neoarchean zircons and display negative Eu and positive Ce anomalies. Thus, rocks of the metarhyolite sill exhibit a Mesoproterozoic thermal process, which triggered the crystallization of a new zircon generation. It should be noted that available petrographic and geochemical data on sill rocks indicate no additional reworking of these rocks. New geochronological data on the Mesoproterozoic thermal event in rocks of the Kostomuksha Greenstone Belt are comparable in age with the late phases of anorthositerapakivi granite magmatism (1547–1530 Ma), which is widespread in the southern portion of the craton along its boundary with the Svecofennian orogen.