| Election Name: | 1921 Caerphilly by-election |
| Type: | presidential |
| Country: | United Kingdom |
| Previous Election: | Caerphilly (UK Parliament constituency)#Elections in the 1910s |
| Previous Year: | 1918 |
| Next Election: | Caerphilly (UK Parliament constituency)#Elections in the 1920s |
| Next Year: | 1922 |
| Election Date: | 24 August 1921 |
| Party1: | Welsh Labour |
| Popular Vote1: | 13,699 |
| Percentage1: | 54.2 |
| Candidate2: | Edmunds |
| Party2: | Liberal Party (UK) |
| Popular Vote2: | 8,958 |
| Percentage2: | 35.5 |
| Party3: | Communist Party of Great Britain |
| Popular Vote3: | 2,592 |
| Percentage3: | 10.3 |
| Map Size: | 250px |
| Posttitle: | Subsequent MP |
| Before Party: | Welsh Labour |
| After Party: | Welsh Labour |
The 1921 Caerphilly by-election was held on 24 August 1921. The by-election was held due to the death of the incumbent Labour MP, Alfred Onions. It was held for Labour by Morgan Jones.[1]
Morgan Jones clinched the Labour nomination despite not being the preferred candidate of the South Wales Miners' Federation. He was the first conscientious objector to be elected to Parliament after World War I.[2]
The Communist Party of Great Britain (CPGB) stood Bob Stewart, a member of its executive committee. The party had been founded in 1920, and this was its first Parliamentary election. It sent almost all its leading figures to campaign in the election.[3]
Jones won an easy victory, with Edmunds in second, and the CPGB a distant third.