The Western star shared the sad news in a statement, where he confirmed his companion's death Thursday evening ... but provided no further details on the circumstances of her passing.

He says ... "Christina was a lovely, caring woman, and I will miss her very much."

Clint and Christina first started dating in 2014 after reportedly meeting at C.E.'s Mission Ranch Hotel and Restaurant in Carmel-by-the-Sea -- a California town where Clint was once the mayor. Christina -- a longtime resident of Carmel-by-the-Sea -- previously worked as a hostess at the historic establishment.

While Clint and Christina were notoriously private about their relationship, CS notably attended the 2015 Academy Awards with her A-list boyfriend. Clint's war drama, "American Sniper," had been nominated for 6 Oscars that evening ... including Best Picture.

Christina was also a fixture on the red carpets for 2016's "Sully," 2018's "The Mule" and 2019's "Richard Jewell."

Prior to his relationship with Christina, Clint had been married twice to model Maggie Johnson from 1953 to 1987 and to news anchor Dina Ruiz from 1996 to 2014. He also had long-term relationships with Frances Fisher and Sondra Locke.

Christina was 61.