| Election Name: | 1918 Wellington South by-election |
| Country: | New Zealand |
| Flag Year: | 1918 |
| Type: | By-election |
| Ongoing: | no |
| Previous Election: | 1914 New Zealand general election |
| Previous Year: | 1914 general |
| Next Election: | 1919 New Zealand general election |
| Next Year: | 1919 general |
| Turnout: | 3,990 |
| Candidate1: | Bob Semple |
| Party1: | New Zealand Labour Party |
| Popular Vote1: | 2,412 |
| Percentage1: | 60.45% |
| Candidate2: | George Frost |
| Party2: | Reform Party (New Zealand) |
| Popular Vote2: | 1,181 |
| Percentage2: | 29.59% |
| Member | |
| Before Election: | Alfred Hindmarsh |
| Before Party: | New Zealand Labour Party |
| After Election: | Bob Semple |
| After Party: | New Zealand Labour Party |
The Wellington South by-election of 1918 was a by-election held in the electorate during the 19th New Zealand Parliament, on 19 December 1918. It was caused by the death of incumbent MP Alfred Hindmarsh, the leader of the Labour Party and was won by fellow party member Bob Semple with a majority of 1,231.[1]
Bob Semple, a miners agent who was previously the organiser of the New Zealand Federation of Labour, was selected as the Labour Party candidate. The nomination was subject of much interest. The unsuccessful aspirants were Tom Brindle, Alec Monteith, John Read, Michael Reardon and Tom Young.[2]
Two Wellington City Councillors also stood as candidates. George Frost and John Castle offered themselves to the electorate, with Frost being endorsed by the Reform Party.
The following table gives the election results: