Lithuania's Independence Act came as a result of World War One and the Bolshevik revolution in Russia.
Lithuanian officials responded with cautious applause, while a photo of the document shared by Professor Liudas Mazylis, who unearthed the act inside the German foreign ministry's archives in Berlin, immediately went viral on social media.
"A Lithuanian-language resolution with all the 20 clearly original signatures was attached next to the German version," Mazylis, a professor at Vytautas Magnus University, told AFP by phone from Berlin.
Lithuania once made up one of medieval Europe's largest military empires but by the late 18th century the Kingdom of Poland and the Grand Duchy of Lithuania were gradually partitioned between neighbouring empires, including Russia.
Lithuania's Independence Act, declared on February 16, 1918, came as a result of World War One and the Bolshevik revolution in Russia.
Lithuanian President Dalia Grybauskaite said it is "likely" that the discovered document is one of the few original copies. The whereabouts of the others have been unknown since World War Two.
"This is extremely joyful news as we look forward to the centenary of the restoration of statehood," Grybauskaite said in a statement.
Interior Minister Eimutis Misiunas called it a "historic day for every patriot" and said it was "a matter of honour to return the birth certificate of the country".
Last month, the Lithuanian business group MG Baltic publicly pledged a million euros to anyone who managed to return the document to Lithuania but Mazylis said he was motivated by the upcoming centenary anniversary, not the money.
After regaining independence from Russia in 1918, Lithuania was annexed during World War II and remained under Moscow's thumb during the Cold War, before becoming the first Soviet republic to declare independence in 1990.