Due to the so-called (3)He shortage crisis, many detection techniques used nowadays for thermal neutrons are based on alternative converters. Thin films of (10)B or (10)B(4)C are used to convert neutrons into ionizing particles which are subsequently detected in gas proportional counters, but only for small or medium sensitive areas so far. The micro-pattern gaseous detector Micromegas has been developed for several years in Saclay and is used in a wide variety of neutron experiments combining high accuracy, high rate capability, excellent timing properties and robustness. We propose here a large high-efficiency Micromegas-based neutron detector with several (10)B(4)C thin layers mounted inside the gas volume for thermal neutron detection. The principle and the fabrication of a single detector unit prototype with overall dimension of ~ 15 × 15 cm(2) and a flexibility of modifying the number of layers of (10)B(4)C neutron converters are described and simulated results are reported, demonstrating that typically five (10)B(4)C layers of 1–2 μm thickness can lead to a detection efficiency of 20–40% for thermal neutrons and a spatial resolution of sub-mm. The design is well adapted to large sizes making possible the construction of a mosaic of several such detector units with a large area coverage and a high detection efficiency, showing the good potential of this novel technique.