In a statement issued on December 19, Ashton said she "regrets" that Minsk officials had sentenced a man to death this week.
A Minsk court sentenced Eduard Lykov, 53, to death on December 17.
He was convicted in November of killing five people.
Ashton said that despite the serious crimes committed, "capital punishment can never be justified."
She added that she hopes Lykov will be able to exercise his right to appeal his sentence. Belarus is the only country in Europe that carries out executions.
It reportedly executed three people last year and currently has three others -- including Lykov -- on death row.