| Competition: | League of Ireland |
| Season: | 1939–40 |
| Winners: | St. James's Gate (2nd title) |
| League Topscorer: | Patrick Bradshaw (29 goals) |
| Matches: | 132 |
| Total Goals: | 540 |
| Prevseason: | 1938–39 |
| Nextseason: | 1940–41 |
The 1939–40 League of Ireland was the nineteenth season of the League of Ireland. Shamrock Rovers were the two-time defending champions.
St James's Gate won their second title.
No new teams were elected to the League.
Cork City withdrew on 13 February 1940. However, a new team, Cork United, were immediately elected in their place, being given special permission to complete Cork City's remaining fixtures.
| Team | Location | Stadium | |
|---|---|---|---|
| Bohemians | Dublin (Phibsborough) | Dalymount Park | |
| Bray Unknowns | Bray | Carlisle Grounds | |
| Brideville | Dublin (The Liberties) | Harold's Cross Stadium | |
| Cork City | Cork | Mardyke | |
| Cork United | |||
| Drumcondra | Dublin (Clonturk) | Clonturk Park | |
| Dundalk | Dundalk | Oriel Park | |
| Limerick | Limerick | Markets Field | |
| St. James's Gate | Dublin (Crumlin) | St. James's Park | |
| Shamrock Rovers | Dublin (Milltown) | Glenmalure Park | |
| Shelbourne | Dublin (Ringsend) | Shelbourne Park | |
| Sligo Rovers | Sligo | The Showgrounds | |
| Waterford | Waterford | Kilcohan Park |