The first Sm-Nd isotope studies of the Carboniferous-Early Cretaceous clastic rocks of the northeastern Siberian Platform have been carried out. Variation in the isotope composition of sediments within this time interval has been determined and interpreted. The high ε_Nd(t) values for Carboniferous-Permian sediments (from -11 to -2) testify to the large contribution of the products of erosion of island-arc and juvenile complexes localized in the Taimyr-Severnaya Zemlya fold-thrust belt in the Carboniferous. The positive ε_Nd(t) values for Triassic sandstones suggest erosion of the coeval igneous rocks of the trap association. The Upper Jurassic and, particularly, Cretaceous sediments of the Lena-Anabar depression and Verkhoyansk foreland basin are characterized by extremely negative ε_Nd(t) values (from -15 to -19), which is evidence for erosion of mature continental crust. The Carboniferous-Triassic complexes of the Verkhoyansk fold-thrust belt, which show higher ε_Nd(t) values, cannot have been a source of detritus for Cretaceous sandstones. The sediments filling the Verkhoyansk foreland basin accumulated as a result of the decomposition of homogenized distal source rocks with the isotope characteristics of the old crust rather than the breakup of the mountain range rising to the east. Salients of the crystalline basement of the Siberian Platform, such as the Aldan Shield, might have been these source provinces.