Artificial soils constructed from wastes and by-products have been considered as a sustainable option for land reclamation. In particular, they could contribute in a complementary manner to natural soils to global climate regulation by storing large quantities of carbon (C). However, the evolution of C stocks in such newly formed soils remains largely unknown. This work aimed at evaluating the dynamics of C in constructed Technosols, focusing on two experimental sites in Lorraine, France, where Technosols were constructed from thermally-treated industrial soil, papermill sludge and green waste compost, and planted with grasses. Soil samples were collected over 12 years, and stocks of C were calculated, taking into account the increase in soil bulk density and the associated decrease in soil thickness over the years. The evolution of the stocks of organic C was compared to the evolution reported for natural grassland soils from the same region and for other Technosols. Initial organic C stocks in the two constructed Technosols were 50% higher than in natural analog soils when calculated over 30 cm, and up to 5 times higher when calculated over 100 cm. Organic C stocks in the two Technosols decreased over the first three years, but increased during the following years, most likely due to the accumulation of plant-derived C. A similar evolution was observed in other planted Technosols. We conclude that Technosols constructed from an adequate set of parent materials can keep large amounts of organic C over time and sequester additional C under permanent plant cover.