The 62-year-old has so far tried to pursue peace with the Marxist rebel group FARC.
"I'm running because I'm convinced we have advanced sufficiently and finally it is possible to reach that future of prosperity and peace all Colombians deserve," Santos said in a televised speech.
Santos is a journalist by trade and Harvard educated and is favored to win re-election over opposition candidate Oscar Ivan Zuluaga.
Zuluaga is against Santos' engagement with FARC. "We will not accept that our soldiers and police keep being murdered or unjustly persecuted while terrorists, kidnappers and murderers walk freely on the beaches in Havana," Zuluaga said following Santos' statement.
Colombians have grown weary of the disputes between FARC and the government. More than 200,000 people have been killed since 1964 in the dispute. FARC has attacked military and civilian targets in the past.