Punjab (/pʌnˈdʒɑːb/ pun-JAHB; Punjabi: pañjāba, pronounced [pəɲˈd͡ʒaːb] ^ⓘ) is a state in northwestern India. Forming part of the larger Punjab region of the Indian subcontinent, the state is bordered by the Indian states of Himachal Pradesh to the north and northeast, Haryana to the south and southeast, and Rajasthan to the southwest; by Indian-administered Jammu and Kashmir to the north and Chandigarh to the east. To the west, it shares an international border with the identically named Pakistani province of Punjab, and as such is sometimes referred to as East Punjab or Indian Punjab for disambiguation purposes. The state covers an area of 50,362 square kilometres (19,445 square miles), which is 1.53% of India's total geographical area, making it the 19th-largest Indian state by area out of 28 Indian states (20th largest, if Union Territories are considered). With over 27 million inhabitants, Punjab is the 16th-largest Indian state by population, comprising 23 districts. Punjabi, written in the Gurmukhi script, is the most widely spoken and the official language of the state. The main ethnic group are the Punjabis, with Sikhs (57.7%) and Hindus (38.5%) forming the dominant religious groups. The state capital, Chandigarh, is a union territory and also the capital of the neighboring state of Haryana. Three of the five traditional Punjab rivers — the Sutlej, Beas, and Ravi — flow through the state.

The history of Punjab has witnessed the migration and settlement of different tribes of people with different cultures and ideas, forming a civilisational melting pot. The ancient Indus Valley Civilisation flourished in the region until its decline around 1900 BCE. Punjab was enriched during the height of the Vedic period, but declined in predominance with the rise of the Mahajanapadas. The region formed the frontier of initial empires during antiquity including Alexander's and the Maurya empires. It was subsequently conquered by the Kushan Empire, Gupta Empire, and then Harsha's Empire. Punjab continued to be settled by nomadic people; including the Huna, Turkic and the Mongols. Punjab came under Muslim rule c. 1000 CE, and was part of the Delhi Sultanate and the Mughal Empire. Sikhism, based on the teachings of Sikh Gurus, emerged between the 15th and 17th centuries. Conflicts between the Mughals and the later Sikh Gurus precipitated a militarisation of the Sikhs, resulting in the formation of a confederacy after the weakening of the Mughal Empire, which competed for control with the larger Durrani Empire. This confederacy was united in 1801 by Maharaja Ranjit Singh, forming the Sikh Empire.

The larger Punjab region was annexed by the British East India Company from the Sikh Empire in 1849. At the time of the independence of India from British rule in 1947, the Punjab province was partitioned along religious lines amidst widespread violence, with the Muslim-majority western portion becoming part of Pakistan and the Hindu- and Sikh-majority east remaining in India, causing a large-scale migration between the two. After the Punjabi Suba movement, Indian Punjab was reorganised on the basis of language in 1966, when its Haryanvi- and Hindi-speaking areas were carved out as Haryana, Pahari-speaking regions attached to Himachal Pradesh and the remaining, mostly Punjabi-speaking areas became the current state of Punjab. A separatist insurgency occurred in the state during the 1980s. At present, the economy of Punjab is the 15th-largest state economy in India with ₹8.02 trillion (US$95 billion) in gross domestic product and a per capita GDP of ₹264,000 (US$3,100), ranking 17th among Indian states. Since independence, Punjab is predominantly an agrarian society. It is the ninth-highest ranking among Indian states in human development index. Punjab has bustling tourism, music, culinary, and film industries.

The Punjab region is noted as the site of one of the earliest urban societies, the Indus Valley Civilisation that flourished from about 3000 B.C. and declined rapidly 1,000 years later, following the Indo-Aryan migrations that overran the region in waves between 1500 and 500 B.C. Frequent intertribal wars stimulated the growth of larger groupings ruled by chieftains and kings, who ruled local kingdoms known as Mahajanapadas. The rise of kingdoms and dynasties in Punjab is chronicled in the ancient Hindu epics, particularly the Mahabharata. The epic battles described in the Mahabharata are chronicled as being fought in what is now the state of Haryana and historic Punjab. The Gandharas, Kambojas, Trigartas, Andhra, Pauravas, Bahlikas (Bactrian settlers of the Punjab), Yaudheyas, and others sided with the Kauravas in the great battle fought at Kurukshetra. According to Dr Fauja Singh and Dr. L. M. Joshi: "There is no doubt that the Kambojas, Daradas, Kaikayas, Andhra, Pauravas, Yaudheyas, Malavas, Saindhavas, and Kurus had jointly contributed to the heroic tradition and composite culture of ancient Punjab." The bulk of the Rigveda was composed in the Punjab region between circa 1500 and 1200 BC, while later Vedic scriptures were composed more eastwards, between the Yamuna and Ganges rivers. The historical Vedic religion constituted the religious ideas and practices in Punjab during the Vedic period (1500–500 BCE), centred primarily in the worship of Indra.

The earliest known notable local king of this region was known as King Porus, who fought the famous Battle of the Hydaspes against Alexander the Great. His kingdom spanned between rivers Hydaspes (Jhelum) and Acesines (Chenab); Strabo had held the territory to contain almost 300 cities. He (alongside Abisares) had a hostile relationship with the Kingdom of Taxila which was ruled by his extended family. When the armies of Alexander crossed Indus in its eastward migration, probably in Udabhandapura, he was greeted by the-then ruler of Taxila, Omphis. Omphis had hoped to force both Porus and Abisares into submission leveraging the might of Alexander's forces and diplomatic missions were mounted, but while Abisares accepted the submission, Porus refused. This led Alexander to seek a face-off with Porus. Thus began the Battle of the Hydaspes in 326 BC; the exact site remains unknown. The battle is thought to have resulted in a decisive Greek victory; however, A. B. Bosworth warns against an uncritical reading of Greek sources who were exaggerative.

Alexander later founded two cities—Nicaea at the site of victory and Bucephalous at the battle-ground, in memory of his horse, who died soon after the battle. Later, tetradrachms would be minted depicting Alexander on horseback, armed with a sarissa and attacking a pair of Indians on an elephant. Porus refused to surrender and wandered about atop an elephant, until he was wounded and his force routed. When asked by Alexander how he wished to be treated, Porus replied "Treat me as a king would treat another king". Despite the apparently one-sided results, Alexander was impressed by Porus and chose to not depose him. Not only was his territory reinstated but also expanded with Alexander's forces annexing the territories of Glausaes, who ruled the area northeast of Porus' kingdom.

After Alexander's death in 323 BCE, Perdiccas became the regent of his empire, and after Perdiccas's murder in 321 BCE, Antipater became the new regent. According to Diodorus, Antipater recognised Porus's authority over the territories along the Indus River. However, Eudemus, who had served as Alexander's satrap in the Punjab region, treacherously killed Porus. The battle is historically significant because it resulted in the syncretism of ancient Greek political and cultural influences to the Indian subcontinent, yielding works such as Greco-Buddhist art, which continued to have an impact for the ensuing centuries. The region was then divided between the Maurya Empire and the Greco-Bactrian Kingdom in 302 B.C.E. Menander I Soter conquered Punjab and made Sagala (present-day Sialkot) the capital of the Indo-Greek Kingdom. Menander is noted for having become a patron and convert to Greco-Buddhism and he is widely regarded as the greatest of the Indo-Greek kings. Greek influence in the region ended around 12 B.C.E. when the Punjab fell under the Sasanians.

Following the muslim conquests in the Indian subcontinent at the beginning of the 8th century, Arab armies of the Umayyad Caliphate penetrated into South Asia introducing Islam into Punjab. In the ninth century, the Hindu Shahi dynasty emerged in the Punjab, ruling much of Punjab and eastern Afghanistan. The Turkic Ghaznavids in the tenth century overthrew the Hindu Shahis and consequently ruled for 157 years, gradually declining as a power until the Ghurid conquest of Lahore by Muhammad of Ghor in 1186, deposing the last Ghaznavid ruler Khusrau Malik. Following the death of Muhammad of Ghor in 1206, the Ghurid state fragmented and was replaced in northern India by the Delhi Sultanate. The Delhi Sultanate ruled the Punjab for the next three hundred years, led by five unrelated dynasties, the Mamluks, Khalajis, Tughlaqs, Sayyids and Lodis. A significant event in the late 15th century Punjab was the formation of Sikhism by Guru Nanak. The history of the Sikh faith is closely associated with the history of Punjab and the socio-political situation in the north-west of the Indian subcontinent in the 17th century.