The sports drama featured Michael B Jordan as Adonis Creed, the son of Apollo Creed, with Stallone reprising the role of Rocky Balboa.
Taking to Instagram, the 71-year-old star posted a still from "Creed", which showed him and Jordan training.
"Getting stronger!!! 2018 #Creed 2 #mgm #fitness #Rocky," Stallone captioned the picture.
"We are continuing to work on developing the sequel to 'Creed'," an MGM spokeswoman told Variety.
"Creed" was directed by Ryan Coogler from a script he co-wrote with Aaron Covington.
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