The decision was an unexpected one. Although President Santos and FARC guerrilla leader Rodrigo Londoño were originally firm favorites to win the prize, Colombia's shock vote to reject the peace deal they brokered cast doubt on the pair's chances.
But despite the result of the plebiscite, the Nobel committee applauded Santos for his efforts to bring "the bloody conflict significantly closer to a peaceful solution”. Here's what you need to know about the Colombia leader:
1. Santos has dedicated his presidency to ending the war with the FARC
Arguably the most notable accomplishment of Santos’s presidency has been his success in bringing the leftist FARC to the bargaining table after decades of conflict. In 2012, the Colombian government initiated direct peace negotiations with the guerrillas for just the third time in history. The start of the talks, which began in Oslo and then continued in Havana, led Santos’s popularity to "spike to roughly 60% approval," Britannica reports.
The talks continued into 2013 and 2014 - the year in which Santos won re-election with just 50.95% of the vote. Over the next couple of years, cease-fires were initiated and disrupted, Santos meeting with the FARC throughout. Eventually, on Sept. 23, it was announced that they had agreed to reach a final peace accord within six months and, after a slight delay, a historic final peace agreement was signed on Sept. 26, in Havana after four years of negotiations, with a pen made from a bullet.
To vote on whether or not to ratify the agreement, Colombians went to the polls on Oct. 2, with forecasts indicating that it would easily be a 'yes' vote. However, the country voted to reject his government’s peace agreement with the FARC by less than 0.5%.
What happens next is uncertain. The FARC say they will honor a ceasefire, which Santos has said will end on Oct. 31.
2. He took office with a landslide victory
The center-right Santos achieved a landslide victory on election day on June 20, 2010, defying predictions of low turnout, thanks to rain and it coinciding with three World Cup games. He took 69% of the votes, against Antanas Mockus of the Green Party's 27.5%.
"In the end, [President Alvaro] Uribe's endorsement, combined with Santos' stay-the-course message, helped produce the biggest rout ever in a Colombian presidential election," TIME reported, when covering the election at the time.
"If we have come so far, it is because we have been standing on the shoulders of giants," Santos told thousands of supporters jammed into a Bogotá sports coliseum. He added that he would rid Colombia of what he described as the "nightmare of violence" and promised to "press forward attacking the enemies of the homeland".
3. His 'Colombian comeback' didn't last long
Santos inherited a bankrupt public-health system, high unemployment and twin battles against drug lords and guerrillas. But in his first year of power his approval ratings were in the '80s and the economy was booming, with strong growth (at 4%) and low inflation; TIME devoted a cover story to him, the tag line reading 'The Colombian Comeback'.
"But most of the impressive growth in Colombia’s economy over the past decade has been due to a combination of commodity exports (mainly oil and coal), construction and domestic consumption," wrote The Economist last year. Indeed, by Santos' third anniversary in the Casa de Nariño, his economic successes were on a downward spiral - partially thanks to drastically decreased revenue coming from oil, Colombia’s largest export product, which began dropping in the second half of 2014. In early 2016, Santos was forced to announce he had cut this year's government budget "with 3% amid an economic slowdown caused by a drop in global oil prices," according to Colombia Reports.
“You have to make adjustments when you have to, even though they are painful. It’s the responsible thing to do,” he said.
4. His domestic policies haven't pleased everyone
Santos' presidency has been riddled with protests, often by people concerned with his domestic policies, mainly education, employment and inflation. "Colombia’s infrastructure is in dire need of investment, and it is hindering economic growth in important sectors like agriculture and tourism," reported Pulsamerica in 2014.
His critics say he has failed to invest in transportation and tourism infrastructure that might give the economy a boost. "Santos needs to recognise the imperative for government-spending in projects that will elevate poor rural areas, rather than those that benefit wealthy urban citizens," the report added. "Santos needs a panoramic view of the country, and should not be too attached to the idea of peace as the sole objective, because it will be a hard quest that needs to be supplemented by strong social infrastructure."
5. He is from one of Colombia's most wealthy and powerful families
Santos' great uncle, Eduardo, was president from 1938 to 1942 and his cousin, Francisco, was Vice President in the Uribe administration. His father edited the country's most important national newspaper, El Tiempo, for over 50 years.
All this has led to accusations that he has been born with a silver spoon in his mouth. "The presidency was his before he was out of short trousers. The first ever election Santos faced was that to become president. It was handed to him on a plate," an article in Colombia-Politics stated, three years after his landslide victory.