Miley Cyrus and Liam Hemsworth made a surprise visit to the Rady Children's Hospital in San Diego on Thursday (29Dec16).

The pair, whose relationship appears to be going from strength to strength following their reunion almost a year ago, stopped by the medical centre with Miley's Happy Hippie Foundation.

While Wrecking Ball singer Miley got into the spirit in a full on hippie ensemble of a bright floral dress, pink fluffy cardigan, purple tights and pale pink go-go boots, Liam kept his look casual in a plaid shirt and jeans.

But his dressed down look didn't stop Miley from officially welcoming her beau into the Happy Hippie fold.

Alongside a picture of herself and Liam with a patient at the hospital, Miley wrote: "Thank you for having us today! Beautiful children everywhere! @liamhemsworth is OFFICIALLY a #happyhippie !!!! @happyhippiefdn."

Following the pair's visit, the Rady Children's Hospital wrote on their official Twitter page: "Thank you for spending the day with us @MileyCyrus and @LiamHemsworth! You brought many smiles to our young patients today!"

Miley founded her Happy Hippie Foundation back in 2014, with the aim of spreading positivity and rallying young people to fight injustice.

She told Out magazine of her intentions with the organisation: "As you get older, you start to realise what’s happening around you. You realize, this isn’t the world I want to be living in. And you say, what do I really want my life to be about?”

Miley also used the Christmas holiday to promote her foundation to her followers on social media, encouraging them to give the "gift of love and acceptance this year to not only their own family but those around the globe who don't get everything they wished for due to life's unfair circumstances!"

She added of the Happy Hippie Foundation, "it's what I am most thankful for this year because of the purpose it gives me. Without it I would feel so lost and useless on this planet!"