Ten-player Reading scored three times in seven second-half minutes to come from 2-0 down and stun West Ham.
The hosts had led thanks to goals from Katharina Baunach and Leanne Kiernan, either side of Rachel Rowe’s sending off deep in first-half injury time.
But throughout the second half, Reading dominated despite being a player short.
Kristine Leine sparked the revival, and Brooke Chaplen fired in seconds later before Jade Moore completed the comeback by poking home from a corner.
West Ham were so shocked by a rampant Reading that it could have been more.
Fara Williams, twice, and Sophie Howard both hit the woodwork in the second half and the home side could not muster any kind of fightback of their own after falling behind late on.