19 (nineteen) is the natural number following 18 and preceding 20. It is a prime number.

19 is the eighth prime number. 19 forms a twin prime with 17, a cousin prime with 23, and a sexy prime with 13. 19 is a strictly non-palindromic number in any base, and a Heegner number. 19 is the maximum number of fourth powers needed to sum up to any natural number (see, Waring's problem). It is the number of compositions of 8 into distinct parts.

The number of nodes in regular hexagon with all diagonals drawn is nineteen.

${\displaystyle {\tfrac {1}{19}}}$ can be used to generate the first full, non-normal prime reciprocal magic square in decimal whose rows, columns and diagonals — in a 18 x 18 array — all generate a magic constant of 81 = 9².

19 is a supersingular prime, a member in the sequence of fifteen such primes that divide the order of the Friendly Giant ${\displaystyle \mathrm {F_{1}} }$. Additionally, 19 also divides the orders of the Janko groups ${\displaystyle \mathrm {J_{1}} }$ and ${\displaystyle \mathrm {J_{3}} }$.

In the Bábí and Baháʼí Faiths, a group of 19 is called a Váhid, a Unity (Arabic: واحد, romanized: wāhid, lit. 'one'). The numerical value of this word in the Abjad numeral system is 19.

19 is a sacred number of the goddess Brigid because it is said to represent the 19-year cycle of the Great Celtic Year and the amount of time it takes the Moon to coincide with the winter solstice.