The Iron-Man star, 54, shared a massive hug with his co-star Emma Thompson, 60, ahead of the special screening at Cineworld Leicester Square on Saturday.
He wore a pair of Doc Martins, a red scalf and dark shades as Thompson dazzled in a chic overcoat, pairing her blonde pixie cut with big dangling earrings.
The duo shared a warm embrace on the red carpet before breaking out into a variety of hilarious animal poses.
Thompson plays Polly the talking parrot in the film while Downey Jr plays Dr Dolitte in the remake of the classic children's film.
She posed alongside a crouched down Robert with her arms outstretched to resemble bird wings.
Downey Jr. joined in on the animistic action by beating his hands on his chest like a gorilla.
His wife Susan Downey also attended the screening to support her husband, as did his other co-star Tom Holland, 23, who plays a long-haired lurcher called Jip in the film.
Instead of embodying the character on the red carpet, Holland brought his pet dog Tess to the screening instead.