The word you are probably seeking is mercenary, which exists both as noun and adjective.
It tends to carry negative connotations and is applied more specifically than in any other context to soldiers of fortune, who fight, not for a national flag, but for financial reward. indeed that is how the term is applied as a noun. A mercenary is such a soldier.
Its etymology is from Latin via Norman French, though the earliest example the OED has, is from Chaucer:
▸ c1387–95 Chaucer Canterbury Tales Prol. 514 He [sc. the parson]
was a shepherde and noght a mercenarye [v.r. mersenarye].
The full range of OED entries for mercenary, without the multiple examples under each sense, is as follows:
1. A person who works merely for money or other material reward; a hireling. In later use (probably influenced also by sense A. 2): a
person whose actions are motivated primarily by personal gain, often
at the expense of ethics. c1387–95—1998
2.a. A person who receives payment for his or her services. Chiefly and now only: spec. a soldier paid to serve in a foreign army or other
military organization. 1523—1974
b. In extended use, with modifying word. 1861—1987
B. adj. 1 a. Of a person, organization, etc.: working or acting
merely for money or other material reward; motivated by self-interest;
materialistic. 1532—1997 b. Of conduct, a course of action, etc.,
or its motivation: characterized by self-interest or the pursuit of
personal gain; prompted by the desire for money or other material
reward; undertaken only for personal gain. 1532—1990
a. Hired, serving for wages. Now: spec. designating a soldier paid to serve in a foreign army or other military organization; (of an
army) composed of such soldiers. 1569—1974 †b. Salaried,
stipendiary; profit-making. Obsolete. 1656—1782
- Of or belonging to a mercenary. a1616—1922