| Lauchlan MacNeill Weir | |
| Honorific Suffix: | MP |
| Office: | Member of Parliament for Clackmannan and Eastern Stirlingshire |
| Term Start: | 14 November 1935 |
| Term End: | 18 August 1939 |
| Predecessor: | James Wellwood Johnston |
| Successor: | Arthur Woodburn |
| Term Start1: | 15 November 1922 |
| Term End1: | 7 October 1931 |
| Predecessor1: | Ralph Glyn |
| Successor1: | James Wellwood Johnston |
| Office2: | Parliamentary Private Secretary to the Prime Minister |
| Alongside2: | Robert Morrison |
| Primeminister2: | Ramsay McDonald |
| Predecessor2: | Charles Rhys |
| Successor2: | Frank Markham, Ralph Glyn, John Worthington |
| Term Start2: | 1929 |
| Term End2: | 1931 |
| Office3: | Parliamentary Private Secretary to the Prime Minister |
| Primeminister3: | Ramsay MacDonald |
| Predecessor3: | Sidney Herbert |
| Successor3: | Sidney Herbert |
| Term Start3: | 1924 |
| Term End3: | 1924 |
| Birth Name: | Lauchlan MacNeill Weir |
| Birth Date: | 1877 |
| Party: | Labour |
| Spouse: | Margaret Gillison (m. 1913) |
| Alma Mater: | University of Glasgow |
| Occupation: | Journalist |
Lauchlan MacNeill Weir (1877–18 August 1939) was a Scottish Labour politician.
He was the son of Robert Weir and was educated at the University of Glasgow. He worked as a journalist and first stood for parliament in Argyllshire in 1918, but was easily beaten by the Coalition Liberal.
He was elected MP for Clackmannan and Eastern Stirlingshire in the general election of 1922, lost his seat in the National Government landslide of 1931, but won it back in 1935, holding on to it until his death in 1939.
MacNeill Weir was appointed Parliamentary Private Secretary to the first Labour Prime Minister Ramsay MacDonald in 1924 and wrote a controversial book entitled The Tragedy of Ramsay MacDonald: A Political Biography published in 1938.
He married Margaret Gillison in 1913. There were no children.
serving alongside Robert Morrison