Radiolarians collected from five localities of the Baltalimani Formation in the Istanbul region are described in detail, and the local stratigraphic ranges and persistence of biostratigraphically important radiolarian species are documented. The spumellarian component is published for the first time, and four new species are described: Albaillella kayai n. sp., Ceratoikiscum jucundum n. sp., Meschedea akcetensis n. sp., and Stigmosphaerostylus istanbulensis n. sp. Two assemblages are recognized: the A. paradoxa and the A. sp. aff. A. undulata-A. deflandrei assemblage. The older A. paradoxa assemblage contains Traenosphaera sicarius, Pararchocyrtium serrensis, Cyrtisphaeractenium mendax, and common to abundant A. paradoxa and Popofskyellium. Albaillella paradoxa is not restricted to the older assemblage but is the only species of Albaillella present. The younger A. sp. aff. A. undulata-A. deflandrei assemblage contains either A. sp. aff. A. undulata or A. deflandrei. Albaillella indensis ambigua, A. kayai n. sp., and Kantollium sp. A occur exclusively in the A. sp. aff. A. undulata-A. deflandrei assemblage, but are rare and patchy in their occurrence. Distinctive taxa common to both assemblages are Archocyrtium lagabriellei, Pylentonema antiqua, C. avimexpectens, C. umbraculatum, C. jucundum n. sp., Trilonche palimbola, M. akcetensis n. sp., S. tortispina, Polyentactinia polygonia, P. aranea, and Cubaxonium? octaedrospongiosum. Many of the taxa present occur in middle and upper Tournasian sections from France, the central Pyrenees, and Germany and provide a solid basis for re-assessing the age of the Baltalimani Formation as middle to late Tournasian (Tn2–Tn3).