The agency will work across all media channels, initially on a 12-month retainer, to promote the brand and attempt to educate consumers, media and trade about personal finance.
The brief also includes creating campaigns for Satsuma to help "shatter common myths" about personal credit.
Satsuma is part of Provident Financial. Smoking Gun already works for Provident’s sister brand, the guarantor loans brand Glo.
Louise O’Garra, head of media and comms at Satsuma, said the firm had already been impressed by the recommendations the agency had made to "bring the brand to life".
Satsuma was launched at the end of 2013 and offers short-term loans between £100 and £1,000.
There was no incumbent in place before the appointment.