| Mathias Mongenast | |
| Office1: | 9th Prime Minister of Luxembourg |
| Term Start1: | 12 October 1915 |
| Term End1: | 6 November 1915 |
| Monarch1: | Marie-Adélaïde |
| Predecessor1: | Paul Eyschen |
| Successor1: | Hubert Loutsch |
| Birth Date: | 12 July 1843 |
| Birth Place: | Diekirch, Luxembourg |
| Death Place: | Luxembourg, Luxembourg |
| Party: | Independent |
Mathias Mongenast (12 July 1843 - 10 January 1926) was a Luxembourgish politician. He was the ninth prime minister of Luxembourg, serving for twenty-five days, from 12 October 1915 until 6 November 1915. He was the director-general (Minister) of Finance from October 1882 to November 1915.
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