Bhutan's King Jigme Khesa Namgyel Wangchuck and his wife Jetsun Pema have revealed the first photo of their baby son - born on Feb 5 - in a Facebook post on Tuesday (Feb 9) night.

The photo, posted on Queen Pema's Facebook page, shows King Wangchuck's father, His Majesty the Fourth Druk Gyalpo, cradling the four-day-old baby, who is wrapped in an orange blanket.

He is flanked by the royal couple, who gaze down lovingly at their child.

"Bhutanese families spent a wonderful Losar today, celebrating the day with family and loved ones," the post read.

"To make the day even more special, we have the honour of bringing to you the very first official photograph of His Royal Highness The Gyalsey."

Losar, also known as the Tibetan New Year, is an important festival in Bhutan, as well as Tibet and for certain ethnic groups in India and Nepal.

Similar to Chinese New Year, it lasts 15 days but the first three days are the most important, and are characterised by dancing, music and general merrymaking.

King Wangchuck, 35, studied at Oxford and in the United States before being officially crowned king in 2008 when his father abdicated two years earlier.

He married Queen Pema, 25, in a fairytale ceremony in 2011. The baby is the couple's first child.