The list of ship decommissionings in 1921 includes a chronological list of all ships decommissioned in 1921.
| Operator | Ship | Class and type | Fate | Other notes | |
|---|---|---|---|---|---|
| 25 July | United States Navy | A-3 | Plunger class submarine | Target ship | Former USS Grampus |
| 1 December | French Navy | Foudre | Seaplane tender and training ship | Stricken and scrapped | Strike date |
| 12 December | Royal Australian Navy | Australia | Indefatigable class battlecruiser | Scuttled in 1924 |
| Ship events in 1921 | |||||||||||
| Ship launches: | 1916 | 1917 | 1918 | 1919 | 1920 | 1921 | 1922 | 1923 | 1924 | 1925 | 1926 |
| Ship commissionings: | 1916 | 1917 | 1918 | 1919 | 1920 | 1921 | 1922 | 1923 | 1924 | 1925 | 1926 |
| Ship decommissionings: | 1916 | 1917 | 1918 | 1919 | 1920 | 1921 | 1922 | 1923 | 1924 | 1925 | 1926 |
| Shipwrecks: | 1916 | 1917 | 1918 | 1919 | 1920 | 1921 | 1922 | 1923 | 1924 | 1925 | 1926 |