The accident happened around 7.30am on Tuesday when Lamyuang Phanthanu, 50, was waiting at the bus stop in front of Major Cineplex Rangsit.
The 1x2 metre board, which was 8m off the ground, fell on the woman, inflicted a severe wound to her head.
She was taken to Pathum Thani Hospital in serious condition.
Police say a pole holding the board was rusty, causing the sign to fall. Police believe the pole had been rusting since the heavy floods in 2011.