This work presents a 0.4-V supply, 12-nW reverse bandgap voltage reference (BGR) with single BJT and indirect curvature compensation. To achieve sub-0.5V operation, the proposed BGR employs the reverse BGR based complementary-to-absolute-temperature (CTAT) voltage generator, which does not suffer from the settling error and thus reducing the biasing current for the BJT. The reduction in BJT biasing current can reduce the power consumption, thus relieving the burden on the 2× charge pump as well, in which the minimum supply voltage of the proposed BGR decreases down to 0.4 V. In this paper, we employ a differential pair proportional-to-absolute-temperature (PTAT) voltage generator to provide a sufficiently large PTAT voltage. To suppress the temperature coefficient (TC) under limited power and voltage budgets, we propose using a nA level PTAT plus nonlinear current to bias both the BJT and PTAT voltage generator. This approach provides an indirect voltage curvature compensation. The proposed reverse BGR fabricated in 65 nm CMOS, occupies an active area of 0.0470 mm². Measurement results from 8 chips show an average reference voltage of 487.4 mV under 0.4 V supply, with an average TC of 28.4 ppm/°C over a temperature range from -40°C to 100°C, while consuming only 12 nW power.