The network is made up of independent vehicle leasing companies in 16 countries across Europe and North America.
Sampo Finance is based in Helsinki and is part of the Sampo Group, one of the biggest banks in Finland.
Finance manager Carl Nykopp said: 'This is an important step for us and we look forward to meeting the needs of multinational customers and tapping into the expertise and power of International Fleet Services to help develop our proposition even further.'
Steve Simpson, HSBC's head of European development, said: 'The network has gained an extremely valuable new member in Sampo Finance and we are delighted with the sophistication and clear customer focus of its proposition. As the network grows the value of global reach backed by local knowledge becomes ever more compelling.'