Tết (Vietnamese: [tet̚˧˦]), short for Tết Nguyên Đán (lit. 'Festival of the first day'), is the most important celebrations in Vietnamese culture. Tết celebrates the arrival of spring based on the Vietnamese calendar, which is mostly based on the lunisolar Chinese calendar and usually has the date in January or February in the Gregorian calendar.

Tết Nguyên Đán is not to be confused with Tết Trung Thu (Mid-Autumn Festival), which is also known as Children's Festival in Vietnam. "Tết" itself only means festival but is often colloquially known as "Lunar New Year" in Vietnamese, as it is often seen as the most important festival amongst the Vietnamese and the Vietnamese diaspora, with Tết Trung Thu regarded as the second-most important.

Vietnamese people celebrate Tết annually, which is based on a lunisolar calendar (calculating both the motions of Earth around the Sun and of the Moon around Earth). Tết is generally celebrated on the same day as Chinese New Year (also called Spring Festival), with the one-hour time difference between Vietnam and China resulting in the new moon occurring on different days. Rarely, the dates of Vietnamese and Chinese Lunar New Year can differ as such in 1943, when Vietnam celebrated Lunar New Year, one month after China. It takes place from the first day of the first month of the Vietnamese lunar calendar (around late January or early February) until at least the third day.

Tết is also an occasion for pilgrims and family reunions. They set aside the trouble of the past year and hope for a better and happier upcoming year. This festival can also be referred to as Hội xuân in vernacular Vietnamese, (festival - lễ hội, spring - mùa xuân).