The identity of the man who worked at the Geneva offices of Mossack Fonseca has been withheld.
His lawyer Thomas Barth told AFP he was "arrested at the start of the month" and was released on Friday provided he agreed "not to leave Switzerland until the investigation is over".
Panama-based Mossack Fonseca has lodged criminal complaints in several countries against people it suspects of being involved in last year's leak of a massive trove of documents on offshore companies.
The company claims the leak was the result of a hack that came from foreign servers.
News of the arrest in Switzerland was first given by the local newspaper Le Temps.
The paper said it had no information on whether he was the anonymous "John Doe" who claimed credit for the unprecedented leaks that toppled the prime minister of Iceland and triggered fraud investigations in several countries.
Though offshore companies are not in themselves illegal, they can be used to engage in illegal activities such as tax evasion or money laundering.