蓝巨星（Blue Giant）是恒星的恒星光谱分类中的第III级，光谱型为O或B型，为巨星的其中一种。蓝巨星拥有极高的亮度，绝对星等一般为-2至-5.5等，表面温度10,000K-55,000K。蓝巨星通常具有相当大的质量，大多数蓝巨星质量在太阳的5到25倍之间，少数O型的蓝巨星质量会更高。蓝巨星具有非常高的亮度，通常蓝巨星的总辐射能量在太阳的800倍(B9.5III)到太阳的10万倍（O8III）之间，一些O2/O3型的蓝巨星亮度则高达太阳的150万倍。蓝巨星拥有较高的表面温度，导致蓝巨星的大小远远不如同光度的黄色/红色巨星。蓝巨星的直径通常在太阳的6-20倍之间.更大的则是蓝超巨星。

Hertzsprung–Russell diagram

Spectral type

Brown dwarfs

White dwarfs

Red dwarfs

Subdwarfs

Main sequence
("dwarfs")

Subgiants

Giants

Bright giants

Supergiants

Hypergiants

absolute
magni-
tude
(M_V)

In astronomy, a blue giant is a hot star with a luminosity class of III (giant) or II (bright giant). In the standard Hertzsprung–Russell diagram, these stars lie above and to the right of the main sequence.

The term applies to a variety of stars in different phases of development, all evolved stars that have moved from the main sequence but have little else in common, so blue giant simply refers to stars in a particular region of the HR diagram rather than a specific type of star. They are much rarer than red giants, because they only develop from more massive and less common stars, and because they have short lives in the blue giant stage.