Sean Penn -- Assistant Injures Herself in Shower Slip and Fall

It was Sean Penn to the rescue today ... when his assistant fell down in the shower and messed up her arm ... and the actor rushed her to urgent care.

Sources close to the actor tell TMZ ... Penn's assistant was taking a shower at Sean's Malibu home when she fell ... and popped out her elbow.

Sean wrapped her up in a towel and took her to a nearby urgent care facility.

Sean and his girlfriend, Shannon Costello, (below) were clearly upset as they waited while the assistant was being treated.