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
Railways 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
See also Route 21 (disambiguation)