Here are some of the key reactions to the award, with many voicing hope it would help boost the peace process after Colombians voted against September's historic accord between the government and the FARC guerrillas after five decades of conflict.
'Peace with social justice'
"The only prize we aspire to is peace with social justice for Colombia, without (right-wing) paramilitary groups, without retaliation (against leftist rebels) or lies," said FARC leader Timoleon "Timochenko" Jimenez.
"Peace in the streets," he wrote on Twitter, after he was notably not named as a co-recipient of the Nobel prize awarded to his one-time arch-foe.
UN chief Ban Ki-moon said the award came "at a critical moment" to give hope and encouragement to the Colombian people.
Ban called the award "a timely message to all who have toiled so hard for peace," adding: "The peace process should inspire our world."
UN refugee chief Filippo Grandi said the prize was recognition of Santos's "political courage".
What about the rebels?
Former FARC hostage Ingrid Betancourt said she believes the guerrilla movement should have shared the prestigious award.
"Yes. It's very hard for me to say yes, but I think so," an emotional Betancourt told French channel I-Tele.
Betancourt said the prize showed "there was no turning back from peace" and was a "just" reward for Santos.
"He was practically alone in fighting for this result. He is making history. He is giving the next generation the possibility of living in a different country. It's a huge moment for Colombia."
Praise from opponent
Colombia's former leader Alvaro Uribe congratulated his rival over the Nobel, despite their sharp disagreement over Santos's efforts to end the civil war.
"I congratulate President Santos on the Nobel," Uribe wrote on Twitter, adding that he hoped the prize would encourage "changes" to the peace accord with the rebels.
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'Come hell or high water' -
"Those who dream of peace always manage to build it, come hell or high water. Very moved that my friend @JuanManSantos has won the #NobelPeacePrize," tweeted EU foreign policy chief Federica Mogherini.
European Parliament President Martin Schulz hailed not only what he described as Santo's "total dedication" to forging peace but also his "calm and collected" reaction to the shock referendum result.