The 21-year-old returned to Portugal last July after a season-long loan move to German lower division side 1860 Munich, and caught the eye of Sporting head coach Jorge Jesus who is looking for reinforcement for his side as they hope to qualify for the Uefa Champions League next season.
"We're delighted to announce the signing of Ghanaian international Lumor Agbenyenu in a deal which is four years and six months," a club statement read.
Agbenyenu thanked his former club for the support and believes he will improve his career at Sporting.
"Thanks to everyone at Portimonense, we had great moments together and I'm grateful for the time. I believe that it was necessary to move and also aim higher in my career," he said.
Agbenyenu, who is line to make his bow this weekend against Estoril, boasts of four international appearances for Ghana after making his debut in 2017.