Giovanni Ribisi -- Low Key Marriage ... Ends with Quiet Divorce

Giovanni Ribisi is ending his marriage the same way he started it -- totally under the radar.

The "Avatar" and "Saving Private Ryan" star filed to divorce actress Agyness Deyn, citing irreconcilable differences in court docs ... obtained by TMZ.

Giovanni filed in January, but according to the docs the couple separated on December 28. They got married in June 2012 with a totally un-Hollywood ceremony -- reportedly doing the deed at the County Registrar's Office.

They don't have kids ... so their main issue will be splitting up property and assets.

This was Deyn's first marriage and Ribisi's second.