Joaquin Phoenix & Michelle Williams -- United In Death

Joaquin Phoenix and Michelle Williams -- both of whom lost loved ones to ODs -- paid their respects Wednesday to Mimi O'Donnell -- the longtime partner of Philip Seymour Hoffman.

Phoenix -- who's brother River Phoenix died from a mixture of heroin and cocaine in 1993 -- paid a visit to O'Donnell's NYC apartment, accompanied by GF Allie Teilz.

Joaquin co-starred with Hoffman two years ago in "The Master."

Williams showed up a short time later. She co-starred with PSH in "Synecdoche, New York" in 2008 ... the same year her ex-partner -- Heath Ledger -- was found dead in his apartment from an accidental overdose of Rx meds.

O'Donnell's been receiving a number of well-wishers ... including Cate Blanchett and Jennifer Aniston's fiance, Justin Theroux.