The first historical record of an attempt to introduce Christianity to Thailand is owed to John Peter Maffei who stated that about 1550 a French Franciscan, Bonferre, hearing of the great kingdom of the Peguans and the Siamese in the East, went on a Portuguese ship from Goa to Cosme (Peguan), where for three years he preached the Gospel, but without any result.
In 1552 St. Francis Xavier, writing from Sancian to his friend Diego Pereira, expressed his desire to go to Siam, but his death on 2 December 1552, prevented him. In 1553 several Portuguese ships landed in Siam, and at the request of the king three hundred Portuguese soldiers entered his service. In the following year two Dominicans, Fathers Hieronymus of the Cross and Sebastian de Cantù, joined them as chaplains. In a short time they established three parishes at Ayutthaya with some fifteen hundred converted Siamese. Both missionaries, however, were murdered by the pagans (1569), and were replaced by Fathers Lopez Cardoso, John Madeira, Alphonsus Ximenes, Louis Fonseca (martyred in 1600), and John Maldonatus (d. 1598).
In 1606 the Jesuit Balthasar de Sequeira at the request of the Portuguese merchant Tristan Golayo, and in 1624 Father Julius Cesar Margico, came to Ayutthaya and gained the favour of the king. A subsequent persecution, however, stopped the propagation of the Christian faith and no missionary entered until Siam was made a Vicariate Apostolic by Pope Alexander VII on 22 August 1662. Soon after, Msgr. Pierre de la Motte-Lambert, Vicar-Apostolic of Cochin China, arrived at Ayutthaya, accompanied by Fathers De Bourges and Deydier. In 1664 he was joined by Msgr. Pallu, Vicar Apostolic of Tong King. Siam, in those days a rendezvous of commercial enterprise in the East, gave shelter to several hundred Annamite and Japanese Christians who had been expelled or lived there as voluntary exiles due to persecutions at home.
Some Portuguese and Spanish Jesuits, Franciscans, and Augustinians had the spiritual care of their countrymen in Siam. Msgr. Pallu, on his return to Rome (1665), obtained a Brief from Pope Clement IX (4 July 1669), by which the Vicariate of Siam was entrusted to the newly founded Society of Foreign Missions of Paris. In 1673 Father Laneau was consecrated titular Bishop of Metellopolis and first Vicar Apostolic of Siam, and ever since Siam has been under the spiritual care of the Society of Foreign Missions. King Phra-Naraï gave the Catholic missionaries a hearty welcome, and made them a gift of land for a church, a mission-house, and a seminary (St. Joseph's colony). Through the influence of the Greek or Venetian, Constantine Phaulcon, prime minister to King Phra-Naraï, the latter sent a diplomatic embassy to Louis XIV in 1684. The French king returned the compliment by sending M. de Chaumont, accompanied by some Jesuits under Fathers de Fontenay and Guy Tachard.
On 10 December 1685, King Phra-Naraï signed a treaty at Louvo with France, wherein he allowed the Catholic missionaries to preach the Gospel throughout Siam, exempted his Catholic subjects from work on Sunday, and appointed a special mandarin to settle disputes between Christians and pagans. However, after the departure of M. De Chaumont, a Siamese mandarin, Phra-phret-racha, instigated a revolution during which the prime minister was murdered, King Phra-Naraï deposed, Msgr. Laneau and several missionaries were taken prisoners and ill-treated, and the Christians were persecuted.
When peace and order were restored in 1690, Bishop Laneau resumed work until his death in 1696. His successor, Bishop Louis of Cice (1700–27), was able to continue it in peace. However, after his death the rest of the century is but the history of persecutions (those of 1729, 1755, 1764 are the most notable), either by local mandarins or Burmese invaders, though the kings remained more or less favourable to the missionaries and to Bishops Texier de Kerlay and de Lolière-Puycontat (1755). During the inroads of the Burmese the Siamese king even appealed to Bishop Brigot for help against the common foe, who sacked and burned the Catholic stations and colleges and imprisoned both the bishop and the missionaries.
In 1769 Father Corre resumed the missions in Siam and thus paved the way for the new vicar Apostolic, Msgr. Lebon (1772–80). However, a fresh persecution in 1775 forced him to leave the kingdom, and neither of his successors, Bishops Condé and Garnault, were able to accomplish much. During the Burmese wars, the Christians were reduced in number from 12,000 to 1000, and Bishop Florens was left in charge with only seven native priests.
It was only in 1826 and 1830 that a fresh supply of European missionaries arrived, among them Fathers Bouchot, Barbe, Bruguière, Vachal, Grandjean, Pallegoix, Courvezy, etc. In 1834 the last was appointed Vicar Apostolic of Siam, and the missions began to revive. Under him Siam numbered 6590 Catholics 11 European and 7 native priests. His successor, Bishop Pallegoix (1840–62), author of "Déscription du royaume Thai ou Siam" and "Dictionnaire siamois-latin-français-anglais" (30,000 words), was one of the most distinguished vicars Apostolic of Siam, the best Siamese scholar, and a missionary among the Laotines. He induced Napoleon III to renew the French alliance with Siam and to send an embassy under M. de Montigny to Siam in 1856. On 8 July 1856, King Mongkut signed a political-commercial treaty with France, by which the privileges granted to the Catholic missionaries by Phra-Naraï in the seventeenth century were renewed. The bishop was highly esteemed by the king, who personally assisted at his funeral and accepted from the missionaries as a token of friendship the bishop's ring.
Thanks to the broad-mindedness of Kings Mongkut (1851–68) and Chulalongkorn (1868–1910), the Catholic Church in Siam enjoyed peace under Pallegoix's successors, Bishops Dupont (1862–72) and Vey (1875–1909). Owing to the complications between France and Siam, in 1894, the missionaries had to endure the ill-will of local mandarins, though the minister of foreign affairs promised that no harm would be done to the missionaries and their work on account of the French invasion.
At the beginning of 20th century, there were about 23,000 Catholic believers, 55 churches and chapels, representatives of such monastic orders, social and educational institutions (e.g. orphanages, schools and a seminary, college). During the 20th century, many other Roman Catholic congregations arrived to work in Thailand.
In 1975 the Catholic Office for Emergency Relief and Refugees was established for protection of moral values and social work, including the urgent problem created by the refugees from Indochina.
From May 10 to 11, 1984 Thailand was visited by Pope John Paul II, the first ever visit of a Pope to Thailand.
On October 22, 1989, the Martyrs of Thailand were beatified. The catechist Philip Siphong Onphitak and six companions had been killed in 1940 under the suspicion of being French spies.
As of 2003, there are 278,000 Catholics in Thailand, which constitutes 0.44% of the total population.
Some of presented in Thailand Roman Catholic orders (Religious of the Good Shepherd, Sisters of the Sacred Heart of Jesus, Sisters of St. Paul de Chartres) take an active part in fight against human trafficking.
The church in Thailand is administrated by 10 dioceses, including two archdioceses.Bangkok (Archdiocese)
Thare and Nonseng (Archdiocese, based in Sakon Nakhon)
Congregations that have worked and/or are working in Thailand include:Ursulines of the Roman Union
Les Filles de la Croix/Daughters of the CrossSisters of Mary Help of Christians
Brothers of St. Gabriel
De La Salle Brothers
Sisters of the Holy Infant Jesus
Oblates of Mary Immaculate
Pontifical Institute for Foreign Missions
Divine Word Missionaries