A legionnaire is a person who is a member of a legion.
In the Federation, the term "legionnaire" most often refers to members of the Federation Security Legion, a group of Starfleet personnel and other police investigators that are required to investigate internal criminal activity for the benefit of the Federation government. In the 2280s decade, Commander Sean Finnegan and Heather Van Horne were security legionnaires. (TOS - Who Killed Captain Kirk? comic: "Finnegan's Wake!")
In an alternate reality, members of a 31st century organization were referred to as "legionnaires". In this reality, the Legion of Super-Heroes was a group sponsored by the United Planets as a peace-keeping organization. (TOS - Legion of Super-Heroes comics: "Issue 1", "Issue 3")