Robert Hjalmar Mellin (19 June 1854 – 5 April 1933) was a Finnish mathematician and function theorist.

He was the oldest among his four siblings, He worked as a translator of his father's religious and literary works after his father's expulsion from the priesthood in 1886 because of his heavy drinking, although he was suspended in 1864 because of the same reason.

Hjarmar's mother Sofia was the sister of the former Councilor of State, Karl Otto Themén (1818-1893).

[3] Mellin studied at the University of Helsinki and later in Berlin under Karl Weierstrass.

In his private life he was known as an outspoken fennoman: a proponent of adopting Finnish as the language of state and culture in the Grand Duchy of Finland, in preference to Swedish, which had predominantly been used hitherto.