The actor attended the glittering Hollywood awards gala with his "This Is Us" co-stars and they celebrated Sterling K Brown's win in the Best Actor in a TV Series, Drama category.
Sterling became the first African-American to win Golden Globe trophy for his role of Randall Pearson on the show.
Ventimiglia later confirmed that he took a plunge while attending the afterparty at the Beverly Hilton in Beverly Hills, California.
"Congratulations Sterling. Excited. And everyone else, great night. Good time at the Globes tonight," Ventimiglia, 40, said in the video after he returned home.
NPR's Eric Deggans tweeted that he saw the actor slip and fall into the uncovered portion of the pool.
"At HBO's Golden Globes party. Believe I just saw Milo Ventimiglia from This Is Us slip and fall into the pool, which is mostly covered but open in one spot..." Deggans tweeted.