Madhesh Province (Nepali: मधेश प्रदेश, romanized: Madhēśa pradēśa) is a province of Nepal in the Terai region with an area of 9,661 km² (3,730 sq mi) covering about 6.5% of the country's total area. It has a population of 6,114,600 as per the 2021 Nepal census, making it Nepal's most densely populated province and the smallest province by area. It borders Koshi Pradesh to the east and the north, Bagmati Province to the north, and India’s Bihar state to the south and the west. The border between Chitwan National Park and Parsa National Park acts as the provincial boundary in the west, and the Kosi River forms the provincial border in the east. The province includes eight districts, from Parsa in the west to Saptari in the east. It is a centre for religious and cultural tourism.

The word madhesh is thought to be derived from the Sanskrit madhya desh (मध्य देश), literally the middle country, which refers to "the central region, the country lying between the Himalaya and the Vindhya mountains". However, in the context of Nepal, Madhesh refers to the region in the Nepal Terai located south of the Siwalik Hills. Madhesh has also been defined as the cultural and linguistic space existing as a basis for identity among the people of the Terai.

Madhesh Province was founded with the provisional name Province No. 2 in September 2015 in accordance with Schedule 4 of the Constitution of Nepal. It received its current name on 17 January 2022, when the provincial assembly declared Janakpur as its capital.

As per Central Bureau of Statistics, Madhesh Province covers about 9,661 km² (3,730 sq mi) of Nepal's total area of 147,516 km² (56,956 sq mi). With 6,114,600 inhabitants as of 2021, it is Nepal's second most populous province. Madhesh Province is surrounded by the Chitwan District to the west, Makwanpur District and Sindhuli District and Udayapur District to the north, Sunsari District to the east, and India to the south. The province has 574,360 ha (1,419,300 acres) of arable land, making it the country's most agriculture-dominated province. The west of the province contains part of Parsa National Park, while part of Koshi Tappu Wildlife Reserve lies within the east. The province is biodiverse, and is crossed by migrating elephants.

According to the 2021 Nepal census, the province has a population of 6,114,600: 3,065,751 males and 3,048,849 females. The province with 20.97% of the country's population has the second-highest population after Bagmati Province in the country, and is the densest province in the country with a density of 633 people per square kilometre.

Maithils are the largest ethnolinguistic group. Yadav is the largest group among the Madhesi people in the province making up around 15.2% of the population. Muslims are the second largest group making up 13% of the population. Teli (5.3%), Kushwaha(koeri) (4.5%), Chamar (4.4%), Kurmi (3.0%), Musahar (2.9%), Dusadh (2.79%), Mallaah (2.26%), Maithil Brahmin (2.2%), and Karan Kayastha (1.5%) are other Madhesi pandit (8.0) groups in the province. Bahun and Chhetri are the largest Khas Arya groups in the province making up 2.34% and 1.99% of the population, respectively. Tharu (5.27%) and Dhanuk (3.49%) is the largest non-Madhesi, non-Khas Arya group followed by Tamang (2.17%).

The Maithili language is spoken by 45.36% of the total population of the province. Bhojpuri is spoken by 18.81% and Bajjika is spoken by 18.44% of the population. Nepali is the province's official language, but is spoken as mother tongue by only 5.76% of the population. Tharu (4.17%), Urdu (4.08%), and Tamang (1.65%) are other languages spoken by a minority of the population.

The Language Commission of Nepal has recommended Maithili, Bhojpuri and Bajjika as official languages of the province, and Urdu, Tharu and Tamang as additional official languages for specific regions and purposes in the province.