The arrest and probe come as The Gambia on Monday welcomed their new president Adama Barrow, who had fled to Senegal after winning a landmark election as Jammeh refused to give up power sparking a political crisis.
Bern prosecutor Christof Scheurer told AFP that Sonko was being investigated under article 264a of the Swiss criminal code, which covers crimes against humanity.
Sonko had been one of Jammeh's top aides, serving in his presidential guard before heading the interior ministry from 2006 to 2016.
Jammeh sacked him in September and Sanko fled to Sweden where his request for asylum was rejected.
Sonko was detained in the Swiss capital Bern following a complaint filed by rights group TRIAL.
"He will be interrogated soon," Amael Gschwind, a spokesman for Bern prosecutors told AFP, confirming the arrest.
TRIAL, which campaigns for the Swiss judicial system to act on crimes committed abroad, described Sonko as one of Jammeh's "strongmen" and claimed he must have been aware of the violations committed under the fallen authoritarian regime.
"Sonko could not have ignored the large-scale torture that political opponents, journalists and human rights defenders suffered," charged Benedict de Moerloose of TRIAL's criminal law division.
According to TRIAL, Sonko arrived in Switzerland in November and applied for asylum.
It was not immediately clear where he had lived between his arrival and his arrest.
TRIAL urged Switzerland to move forward with prosecution, suggesting it could give positive momentum to the unprecedented political developments under way in The Gambia.