In 2006, on the 60th anniversary of him taking the throne — and the following year on his 80th birthday — they wore their hearts on their sleeves.
The mass of yellow shirts was inspired by the Thai colour for Monday — the day he was born. It soon became the symbol of royalist protests against then prime minister Thaksin Shinawatra and his red-shirted supporters.
When Mr Thaksin was ousted as Prime Minister in a 2006 military coup, many believed it had the king's blessing.
Indeed, even the most recent coup — which reinstated military rule in May 2014 — was seen by many as a deliberate act to control the monarchy and the succession, not to mention the royal fortune, at a time when Thailand will likely face a power vacuum.
"It's about who will be in power when the royal succession takes place," Ernest Bower, senior adviser at the US Centre for Strategic and International Studies, said.
"It's like a musical chairs game. When the music stops — when the king dies — whoever has power gets to organise the next steps," he told CNBC News in the months after the coup.