Berlin-based dissident Liao Yiwu, who is expected to welcome her in the German capital later Tuesday, voiced his joy on Twitter, saying: "I am so, so, so happy!"
The couple's supporters often say that Liu Xia, who has never been formally charged, is guilty of nothing but the "crime" of being Liu Xiaobo's wife.
While there was no immediate comment from its embassy, Germany had been urging China to allow Mrs Liu to leave.
This February during the Lunar New Year holiday, Liu Xia and her brother Liu Hui were visited by a top official from the Ministry of Public Security, trusted by President Xi Jinping, who suggested she could be freed after the annual legislative session the following month, sources told the South China Morning Post earlier.
China sentenced Liu Xiaobo in December 2009 to 11 years' imprisonment on charges of inciting subversion of state power after he helped write a manifesto calling for political and economic liberalization.
"The lesson of Liu Xia and her courageous husband, the late Nobel Peace Prize victor Liu Xiaobo, who was murdered by the Communist Chinese regime, is simple yet powerful: darkness fears light and mendacity fears truth, so tyrants can not tolerate either", he continued. Liu Xiaobo was a writer and a poet imprisoned for advocating for broad political reforms and greater human rights in China.
Hua Chunying, a Chinese foreign ministry spokeswoman, said Liu's travel to Germany for medical treatment was "of her own free will".
Friends and family quoted in media reports say she boarded a Finnair flight in Beijing on Tuesday for Europe.
Liu is a poet and artist.
Liu, as has been noted multiple times, is guilty of only having fallen in love with Liu Xiaobo, who she married in 1996.
"I want to marry that enemy of the state!" she said shortly before the pair Wednesday in 1996 during Liu Xiaobo's stay at a labour camp, according to a biography of the dissident by Yu Jie. He returned from an academic job in the United States to take part in the 1989 demonstrations in Tiananmen Square and was arrested in the aftermath of the massacre there - the first of several stints in Chinese jail cells. People who know about the issue said Merkel also discussed Liu's case during a visit in May.
In May, Berlin officials again said that should Liu Xia choose to go to Germany, she would be welcome there. The last time China let a high-profile political prisoner leave was in 2012, when blind activist Chen Guangcheng was allowed to fly to NY after escaping from house arrest and hiding for six days in the U.S. Embassy in Beijing. "Dying is easier than living - there is nothing simpler for me than to protest with death".
He died last summer of late-term liver cancer after the Chinese government repeatedly denied his request to receive medical treatment overseas.
Chinese authorities had consistently maintained Liu was free but imposed severe restrictions on her movement and kept her under constant surveillance. "She has been an unwilling symbol of the brutality of China's treatment of human rights activists". "We couldn't save Liu Xiaobo, which is a regret that I will carry for the rest of my life, and we wouldn't have been able to bear it if anything had happened to his widow".
Diplomats have said that authorities had continued to monitor Liu Xia after the death of her husband and she had only been able to meet and speak to friends and family in pre-arranged phone calls and visits.