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, seconds later Brooke Chaplen fired in and 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.