The Earl, David Armstrong-Jones, is 21st in line to the throne and has two grown-up children with wife Serena, the Countess of Snowdon.
“The Earl and Countess of Snowdon have amicably agreed that their marriage has come to an end and that they shall be divorced,” the BBC quoted a spokesman for the couple as saying.
The Earl, who has a furniture company under his professional name David Linley, is the son of the queen’s sister Princess Margaret, who died in 2002, and photographer Lord Snowdon, who died in 2017.
The news comes after a tough few months for the queen.
Prince Harry and wife Meghan plunged the royal family into crisis last month when they announced they were cutting back on official duties.
The queen’s second son, Prince Andrew, stepped down from public duties in November over his connections to U.S. financier Jeffrey Epstein, who was facing charges of trafficking underage girls for sex when he killed himself last year.
Earlier in February, the queen’s eldest grandson, Peter Phillips, and wife Autumn, said they were divorcing after 12 years of marriage.