The soap star reportedly phoned an ex-girlfriend in the early hours of the morning, before police officers discovered his body later in the day.
Santino's death came just hours after he was forced to put his beloved pet dog down on Tuesday (24Jan12) - the day of the star's 47th birthday.
The Gossip Girl actor was reportedly put under pressure to put pit bull Rocco down by the building manager where he lives after tenants complained about his barking.
And Santino was so distraught over the pooch's death, he took his own life the following day, reports the New York Post.
Leaving behind a suicide note, Santino wrote, "Today I betrayed my best friend and put down my best friend. Rocco trusted me and I failed him. He didn't deserve this."
The dog was cremated and the actor's pal, Stuart Sarnoff, reveals Santino's remains will be too: "One way or another, their ashes will be together forever."
Police officials suspect he died of a drug overdose.