The acquisition, which Mediobanca expects to boost its fee income by 10%, comes ahead of a new three-year strategy CEO Alberto Nagel will unveil on May 24.
In the same year, it advised Germany's Aareal Bank (ARLG.DE) on the sale of a stake in its $1.1 billion software unit Aareon to Advent International.
The deal will boost earnings per share, based on last year's figures, Mediobanca said, without elaborating.
With 86 staff and some $100 million in yearly revenue, Arma was founded in 2003 by Paul-Noël Guély, a former head of software and services investment banking at Goldman Sachs.
Arma has offices also in Munich, a U.S. presence and a network of affiliated advisory firms in Japan, Australia, Israel, Turkey and Brazil.