Balthazar Mathias Keilhau (2 November 1797 – 1 January 1858) was a Norwegian geologist and mountain pioneer.

He is regarded as the founder of the discipline of geology in Norway, and has also been credited for the discovery of the Jotunheimen mountain range.

[1] His wife had been engaged to mathematician Nils Henrik Abel, a friend of Keilhau.

When Abel died of tuberculosis in 1829, Keilhau was worried about his fiancé, and offered to marry her, although they had never met, and she accepted.

[1] The 1820 journey, which he made along with fellow student Christian Boeck, was thoroughly documented in the article "Nogle efterretninger om et hidtil ubekendt stykke af det söndenfjeldske Norge".