"At the same time, they are representatives of all journalists who stand up for this ideal in a world in which democracy and freedom of the press face increasingly adverse conditions," she added.
The prize is the first for journalists since the German Carl von Ossietzky won it in 1935 for revealing his country's secret post-war rearmament programme.
"Free, independent and fact-based journalism serves to protect against abuse of power, lies and war propaganda," Reiss-Andersen said.
Muratov is editor-in-chief of Russian investigative newspaper Novaya Gazeta, which has defied the Kremlin under president Vladimir Putin with probes into wrongdoing and corruption, and extensively covered the conflict in Ukraine.
He is the first Russian to win the Nobel Peace Prize since Soviet leader Mikhail Gorbachev -- who himself helped set up Novaya Gazeta with the money he received from winning the award in 1990.
Ressa heads Rappler, a digital media company which she co-founded in 2012, and which has grown prominent through investigative reporting, including into large scale killings during a police campaign against drugs.
"I am in shock," Ressa told a live broadcast by Rappler.
In August, a Philippine court dismissed a libel case against Ressa, one of several lawsuits filed against the journalist who says she has been targeted because of her news site's critical reports on president Rodrigo Duterte.
The plight of Ressa, one of several journalists named Time Magazine Person of the Year in 2018 for fighting media intimidation, has raised international concern about the harassment of media in the Philippines, a country once seen as a standard bearer for press freedom in Asia.
In Moscow, Nadezhda Prusenkova, a journalist at Novata Gazeta, told Reuters staff were surprised and delighted.
“We’re shocked. We didn’t know,” said Prusenkova. "Of course we’re happy and this is really cool."
The Kremlin itself congratulated Muratov on the award.
"He persistently works in accordance with his own ideals, he is devoted to them, he is talented, he is brave," said spokesman Dmitry Peskov.
The award will give both journalists greater international visibility and may inspire a new generation of journalists, said Dan Smith, director of the Stockholm International Peace Research Institute.
"We normally expect that greater visibility actually means greater protection for the rights and the safety of the individuals concerned," he told Reuters.
The Nobel Peace Prize will be presented on 10 December, the anniversary of the death of Swedish industrialist Alfred Nobel, who founded the awards in his 1895 will.