The El Tiempo daily reported Saturday several of the accessed emails were related to the possible, but never completed, purchase of some works of art.
Others were said to deal with school tuition payments for his daughter, Maria Antonia.
In response to the hacking, the government has taken additional electronic security measures.
"My personal email account and those of certain members of my family were intercepted, a criminal activity that I consider without hesitation to be serious, unusual and unacceptable," Santos said in a statement yesterday.
"In the coming days and until the campaign is over, additional information may be brought forward in an attempt to tarnish my good name and that of my government," Santos added.
The revelations come amid other reports that an army intelligence unit had spied on the Colombian government's negotiating team at peace talks with FARC rebels in Cuba.
The talks, which began more than a year ago in the capital Havana, aim to put an end to the longest-running conflict in Latin America, which has left hundreds of thousands dead in over a half century.
