The couple - who are on holiday at the plush Telluride Ski Resort - were whizzing down the slopes when they spotted a woman in distress.
The pair abandoned their run and hurried over to help the 27-year-old American woman, checking that she was comfortable and stayed with her until emergency services arrived.
An onlooker told Britain's Daily Star newspaper: "Justin and Cameron had been on the slopes and were going down a red run when they saw someone bang
into this woman. And she went flying. Everyone just looked on shocked as she landed in a heap on the ground screaming with pain and looked as though she'd broken her legs.
"No-one had any idea the couple who dashed to help her were Cameron Diaz and Justin Timberlake until 20 minutes later when they took their hats and masks off to meet the paramedics who'd turned up."
The source added: "They were amazing and you'd never have thought they were so famous."