Since 2016, 431 activists have been killed, the majority coming from the indigenous, black and peasant communities, the ombudsman said.
One human rights activist or social leader was murdered every two days in Colombia last year, a 36.5 percent increase on 2017, the ombudsman said on Thursday.
"In 2018 there were 172 murders of social leaders and human rights activists," the Defensoria del Pueblo (people's defence) said in a statement.
That was 46 more than in 2017 in a country wracked by a half-century of multi-faceted armed conflict involving Marxist guerrillas, drug-traffickers, paramilitaries and the armed forces.
Since 2016, the ombudsman says 431 activists have been killed, the majority coming from the indigenous, black and peasant communities.
Ombudsman Carlos Alfonso Negret expressed his "concern" about the level of risk faced by activists and denounced the fact that these crimes "were committed in full view of all the authorities."
Things haven't improved in early 2019 either, with the ombudsman reporting seven such murders between January 1-8.
Right-wing President Ivan Duque's government has vowed to implement measures to prevent these crimes, which sparked mass street protests last year.
Last month, the National Indigenous Organization said that since Duque took office in August, violence against their leaders has surged.