Sedimentary time markers based on the recent history of anthropogenic radionuclides or pollutants are extremely valuable in synchronizing and calibrating proxy-based records with instrumental data and observations. However, such time markers are rare for the early 20th century, and any additional time marker for this time period would be valuable for studying the early anthropogenic impact on Earth. The invasive marine phytoplanktonic diatom Pseudosolenia calcar-avis (Schultze) (Sundström, 1986) was first observed in the Black Sea in A.D. 1924–1926 after its introduction by ships’ ballast water. In a sediment core dated by an event stratigraphy approach, the first appearance of both remnant frustules and two C25 highly branched isoprenoid (HBI) alkenes produced by P. calcar-avis has been estimated at A.D. 1925 ± 8 yr. Therefore, the appearance of P. calcar-avis frustules and C25 HBIs can serve as a time marker (or isochrone) for A.D. 1925 in Black Sea sediments. The pronounced increase in C25 HBI contents toward the present day reflects, very likely, the increase in the biomass of P. calcar-avis, which became a mass phytoplankton species in the modern Black Sea. Using anthropogenic radionuclides and pollutants, together with first appearances of invasive species, as sedimentary time markers for the last century is a promising stratigraphic approach that could be applied in other suitable environments where historical data on invasive species are available.