The actor, however, has not commented if the tattoo is permanent or temporary.
The ink read "Times Up" on her arm, missing the apostrophe for the organisation.
Watson, 27, has been an outspoken proponent of the Time's Up movement, which began after bombshell exposes revealed decades of alleged sexual misconduct by Hollywood producer Harvey Weinstein.
The movement calls on people who have experienced sexual misconduct in the workplace to speak up. A number of Hollywood stars, including Watson, have voiced their support and donated to the movement.