The group announced last week that it would take over the licence of the Lampre-Merida team.
Costa, who won the world championships in 2013, said being part of the project was special for him.
"I had received proposals from other teams, however when Saronni described to me his ideas for the future and the project, I had no doubts and I immediately chose to sign a new contract for the next two years," the 29-year-old said in a statement released by the team. "I really appreciate that Saronni and the group of TJ Sport trust in me. This is a great motivation for me."
Ulissi, 27, has raced since 2010 with Lampre-Merida, the only professional team he's competed with so far in his career. He won two stages of the Giro d'Italia in May and recently took the overall at the Czech Cycling Tour.
"I'm happy to continue along the professional path I have started in the team several years ago, adding a new enthusiasm which comes from the Chinese project," he said. "I'm sure the next two years will be full of satisfactions."
The new formulation of the Italian team was announced at a press conference last week in Beijing. Chinese company TJ Sport Consultation was announced as the new licence holders of Lampre-Merida. TJ Sport Consultation replaces CGS Cycling as the holding agent to create China's first WorldTour team from the 2017 season onwards. A Chinese naming rights sponsor is to be announced later.
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