The seven men and one woman were seized by gunmen believed to belong to the ELN in Novita.
"The eight kidnap victims... have been freed, thanks to public pressure," Santos wrote on Twitter.
The seven men and one woman were seized by gunmen believed to belong to the ELN in Novita, a small town in the Choco region, the poorest part of the country in northwestern Colombia.
Recent hostage-takings by the ELN, whose initials stand for National Liberation Army, come with the leftist guerrilla group and the government poised to begin peace talks after decades of hostilities.
Colombia's government and the ELN, the country's last active rebel force, are due to resume talks on May 16 in Ecuador after initiating negotiations in February.
In November, a larger rebel group, the Revolutionary Armed Forces of Colombia (FARC), signed a peace deal with the government after four years of talks.
The ELN has an estimated 1,500 fighters, compared with the FARC's 7,000.