"His non-violent resistance made him a hero in the battle for democracy and human rights. RIP," Maas wrote on Twitter.
Germany had said it was prepared to welcome Liu for medical treatment, after he was transferred from prison to hospital last month following a terminal liver cancer diagnosis.
But the Chinese government rejected international calls to allow the longtime campaigner and his family to leave its borders.
Liu became the first Nobel Peace Prize laureate to die in custody since German pacifist Carl von Ossietzky, who passed away in a hospital while held by the Nazis in 1938.
Liu was arrested in 2008 after co-writing Charter 08, a bold petition that called for the protection of basic human rights and reform of China's one-party Communist system.