| Office: | Senator |
| Term Start: | 1 June 1973 |
| Term End: | 27 October 1977 |
| Term Start1: | 23 June 1965 |
| Term End1: | 5 November 1969 |
| Constituency1: | Industrial and Commercial Panel |
| Term Start2: | 5 November 1969 |
| Term End2: | 1 June 1973 |
| Term Start3: | 14 December 1961 |
| Term End3: | 23 June 1965 |
| Constituency3: | Nominated by the Taoiseach |
| Term Start4: | 9 February 1960 |
| Term End4: | 14 December 1961 |
| Constituency4: | Administrative Panel |
| Birth Date: | 1901 |
| Birth Place: | County Monaghan, Ireland |
| Death Date: | 6 August 1977 |
| Death Place: | County Monaghan, Ireland |
| Nationality: | Irish |
| Party: | Fianna Fáil |
| Branch: | Irish Republican Army |
John J. Brennan (1901 – 6 August 1977)[1] was a draper and publican, Fianna Fáil politician and a member of Seanad Éireann from 1960 to 1977.[2]
He was from Clontibret in County Monaghan and was active in the Irish Republican Army during the Irish War of Independence.[2] He was elected to the County Council in 1942.[2] In Castleblayney, he was chair of the urban district council and president of the chamber of commerce.[2] He stood unsuccessfully for Dáil Éireann in Monaghan in the elections of 1948,[2] [3] 1954,[2] [3] and 1957.[3] He was a friend and early political backer of Erskine H. Childers, Monaghan TD and later President of Ireland.[2]
He was elected to the 9th Seanad on 9 February 1960 at a by-election to the Administrative Panel[1] caused by the death of John O'Leary.[4] He was elected to the 11th (1965) and 13th (1973) Seanad from the Industrial and Commercial Panel,[1] and was nominated by the Taoiseach to the 10th Seanad in 1961 and to the 12th Seanad in 1969.[1]