Line 21 may refer to:
Closed Captioning
- EIA-608, a standard for closed captioning for NTSC TV broadcast, also known as "line 21 captions"
Telecommunications
Line 21, a Hungarian telecommunications company Vodafone Hungary
Railway Line
In operation
- Line 21 (Guangzhou Metro), a metro line of the Guangzhou Metro in China
- Line 21 (Zürich), Switzerland
- Yangluo Line, a metro line of the Wuhan Metro in China
Under Planning
- Line 21 (São Paulo Metro), a future metro line of the São Paulo metro
- Line 21 (Shanghai Metro), a future metro line of Shanghai Metro