Pathum Wan (Thai: ปทุมวัน, pronounced [pā.tʰūm.wān]) is one of the fifty districts (khet) of Bangkok, Thailand. It lies just beyond the old city boundary of Khlong Phadung Krung Kasem and was a rural area on the eastern outskirts of the city when royal villas were built there in the late nineteenth century. The district was officially established in 1915, and covers an area of 8.37 square kilometres (3.23 sq mi). A large part of the district area is taken up by the campus of Chulalongkorn University and the green expanses of Lumphini Park and the Royal Bangkok Sports Club. By the turn of the 20th–21st centuries, the district had become known as the modern-day city centre, home to the prominent shopping areas of Siam and Ratchaprasong.

When King Rama I established Bangkok as his capital in 1782, he had canals dug including Khlong Maha Nak, which extended eastward from the fortified city proper of Rattanakosin Island. Communities formed along its bank, including Ban Khrua, a Muslim community mainly of Cham settlers, which stood at the end of the canal. As the canal was extended further eastward and became known as Khlong Bang Kapi (later Khlong Saen Saep), the outlying areas were cleared for paddy fields, though apart from the canal's banks the areas remained sparsely populated.

By the reign of King Mongkut (Rama IV, 1851–1868), the city had spread beyond its original walls, and Khlong Phadung Krung Kasem was dug to extend the city's periphery in 1852. The eastern fringe of the city was then dominated by expansive rice fields, with Thung Bang Kapi occupying the areas along Bang Kapi Canal. The area that is now Pathum Wan was occupied by royal paddy fields by the canal, between Khlong Phadung Krung Kasem and Ban Khrua to the west and the Bang Kapi fields to the east. Some Lao war captives had also been settled in the area following the razing of Vientiane in 1827.

Sometime before 1855, Mongkut had part of the royal fields south of the canal landscaped with ponds and waterways, which were planted with decorative lotuses, for leisurely visits. A royal villa was built there, which was named Pathum Wan, meaning "lotus forest" (from Pali paduma + vana). A temple, named Wat Pathumwanaram, was also built next to the palace. The King made annual visits to the palace and the temple, at first travelling by boat. A road was later built, continuing eastward from Bamrung Mueang Road, linking the palace to the city. It was originally known as Sa Pathum or Pathum Wan Road before being renamed Rama I Road in 1920.

With the royal development, the area began to attract more residents, and more palaces were built during the following reign of King Chulalongkorn (Rama V, 1868–1910). The King had a palace built for Prince Svasti Sobhana west of Pathum Wan Palace in 1880, and commissioned Windsor Palace in 1881 for his own son Prince Vajirunhis, buying up large amounts of land for what was to be its extensive grounds. He also later gifted a plot of land to another son, Prince Mahidol Adulyadej, though it was Mahidol's mother Queen Savang Vadhana who oversaw the construction Sa Pathum Palace there after Chulalongkorn died in 1910.

Windsor Palace had become vacant following Vajirunhis's death in 1895, and its grounds became the site of several successive educational institutions which became established as Chulalongkorn University in 1917 by King Vajiravudh (Rama VI, r. 1910–1925). By then, several canals, roads and railways had been built through the area. West of the university grounds the Bangkok Railway Station, the country's main terminus, had been built in 1916. To the east, the Phloen Chit area was developed by the enterprising businessman Nai Lert around the same time. When the administration of Bangkok's districts was reorganized in 1915, the district was established as Amphoe Pathum Wan, bounded by Khlong Saen Saep to the north, the Chong Nonsi Railway to the east, Khlong Hua Lamphong to the south, and Khlong Phadung Krung Kasem to the west. Its status, along with Bangkok's other districts', was later changed from amphoe to khet in 1972.

Pathum Wan District saw increasing development throughout the 20th century, especially since the 1970s when the area along Rama I and Phloenchit roads became developed as a major shopping and commercial district. The Erawan Hotel opened on Ratchaprasong Intersection in 1956, and the Thai Daimaru department store, famous for introducing air conditioning and the escalator, opened on the opposite corner in 1964. Siam Square was developed as a low-rise retail area by Chulalongkorn University the same year, and the Siam Inter-Continental Hotel opened opposite it in 1966. Rajadamri Arcade—a modern, enclosed shopping mall—opened at Ratchaprasong in 1972, and Siam Center (next to the Inter-Continental) followed the next year. Gradually, Siam and Ratchaprasong supplanted the Wang Burapha area as Bangkok's centre of urban retail and youth culture. With the opening of the BTS Skytrain in 1999, the location of its central interchange at Siam station helped cement the neighbourhood's prominence as Bangkok's modern-day city centre. The area's prominence led to its occupation by anti-government protesters in 2010, which ended in a violent military crackdown.

Pathum Wan District occupies 8.369 square kilometres (3.231 sq mi) in Bangkok's Phra Nakhon side (east of the Chao Phraya River). It is bordered by Dusit and Ratchathewi districts to the north across Khlong Saen Saep, Watthana and Khlong Toei to the east across the Maenam railway, Sathon and Bang Rak to the south across Rama IV Road, and Pom Prap Sattru Phai to the west across Khlong Phadung Krung Kasem.