Pope Francis met with the Supreme Patriarch on Thursday in a gilded temple on the first full day of his Asia tour aimed at promoting religious harmony.
This is the pontiff's first visit to Buddhist-majority Thailand -- where just over 0.5% of the population are Catholics -- before he jets off to Japan on Saturday.
He is pushing a message of inter-faith peace on a four-day visit that will see him lead a mass later Thursday for tens of thousands of faithful from across Southeast Asia.
The 82-year-old head of the Catholic church also delivered impassioned remarks about the plight of vulnerable children and women who he said deserved a "dignified" future.
In a highly symbolic meeting on Thursday, he sat down with the Supreme Patriarch at Wat Ratchabophit.
"Catholics have enjoyed freedom in religious practice, despite their being in a minority, and for many years have lived in harmony with their Buddhist brothers and sisters," the Pope said in a speech at the meeting.
The pair sat before a brilliant gold Buddha statue inside the ornate temple -- the Supreme Patriarch barefoot and draped in orange robes as they spoke.
The Pope reciprocated the gesture, removing his shoes for part of the tete-a-tete.
In an earlier speech, the Pope said the meeting was "a sign of the importance and urgency of promoting friendship and inter-religious dialogue".
It was the same temple visited by John Paul II on the last papal trip to Thailand in 1984.
This visit coincides with the 350th anniversary of the founding of the "Mission de Siam", marking the first visit by Catholic missionaries who arrived from Europe in the 17th century.
Though Christianity's first visitors were initially met with scepticism, today Thailand's nearly 400,000 Catholics face little discrimination and the country is largely free of religious conflict.
Message for migrants
The head of the world's 1.3 billion Catholics met with Prime Minister Prayut Chan-o-Cha at a red carpet welcome ceremony at Government House, before addressing medical staff at Saint Louis Hospital.
He was accompanied throughout the day by his cousin Sister Ana Rosa, who has lived in Thailand for decades and is helping the pontiff as a translator.
At the hospital, he praised the valuable service "the Church offers to the Thai people, especially to those most in need".
Earlier he made a plea for the women and children "who are wounded, violated and exposed to every form of exploitation, enslavement, violence and abuse", calling for a "dignified future" for the youth.
Later Thursday the Pope led a huge mass for tens of thousands of people, including ethnic Karen Christians from northern Thailand and Vietnamese Catholic refugees living in Bangkok at the Supachalasai National Stadium.
He also met His Majesty the King, before kicking off another full day Friday that will see him greet Catholic leaders and host a second mass.
On Saturday he flies to Japan, where he will visit Hiroshima and Nagasaki, two cities devastated when the US dropped atomic bombs at the end of World War II in 1945.
The Pope, who years ago had hoped to be a missionary in Japan, has made strong calls for a ban on nuclear weapons.
Since his election six years ago, the Pope has made two trips to Asia, visiting the Philippines and Sri Lanka in 2014, followed by Myanmar and Bangladesh in 2017.
