Kate Middleton is starting off her new year by giving back to the hospital where she received her cancer treatment ... paying a surprise visit to the medical center in a rare public outing.

Check it out -- the Princess of Wales stopped by the Royal Marsden Hospital in London Tuesday, where she spoke with cancer patients about their respective health journeys ... something she is all too familiar with.

Kate also made time for the hospital's staffers, who she thanked for their service, especially after her own battle with cancer.

The princess indicated she was even ready to open up further about her health struggle ... noting it was "nice" to use the front door of the hospital after "having made so many quiet, private visits."

While the British royal was once known for her many public appearances and charitable work, she notably took a large step back last year during her health crisis ... later revealing she underwent chemotherapy to combat a cancer diagnosis.

She's never shared the type of cancer she overcame.

Princess Kate confirmed she had reached the end of her chemotherapy treatment in September, however ... and has slowly but surely returned to the spotlight in the last several months.

Her visit to the Royal Marsden Hospital is certainly a big indicator that the future Queen of England is ready to get back to work in 2025. Welcome back!!!