北落师门（英语：Fomalhaut / α PsA / 南鱼座α）是南鱼座的主星，距离地球约25.1光年。在地球上的视星等为1.16，是除太阳外，在地球上能看到的第17位亮星。

北落师门是三合星，主星北落师门A的光谱分类为A3V，直径约为太阳直径的1.7倍，质量约为2.3倍，亮度15倍。它只有约2到3亿年的年龄，是非常年轻的恒星。伴星北落师门B，即南鱼座TW，是一颗光谱K型恒星。另一颗伴星LP 876-10则是光谱M型恒星。

在北落师门周围，距离北落师门133至158天文单位的地方，围绕着一圈圆盘状尘埃云。1998年，人们通过观测和推测，认为尘埃云中很可能已经产生了行星。

Fomalhaut (Alpha Piscis Austrini, Alpha PsA, α Piscis Austrini, α PsA) is the brightest star in the constellation Piscis Austrinus and one of the brightest stars in the sky. It is a class A star on the main sequence approximately 25 light-years (7.7 pc) from Earth as measured by the Hipparcos astrometry satellite. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. It is classified as a Vega-like star that emits excess infrared radiation, indicating it is surrounded by a circumstellar disk. Fomalhaut, K-type star TW Piscis Austrini and M-type star LP 876-10 constitute a triple system even though the companions are separated by several degrees.