The 28-year-old Spaniard has spent his whole career at Barcelona until now, winning every major honor.
Chelsea is paying a transfer fee of 30 million euros ($33 million), Barcelona said, with 3 million euros dependent on his performances at the London club.
"I am very excited to start my adventure at Chelsea and I am here to keep winning titles," Pedro said.
Chelsea has made a stuttering start to its season, losing the Community Shield to Arsenal and opening its Premier League title defense with a draw and a loss.
Chelsea did not announce contract details for Pedro, who can play as a winger or center forward.
Chelsea's first-choice striker Diego Costa has been troubled by hamstring problems, while backup Radamel Falcao is yet to prove himself in the Premier League after a disappointing stint at Manchester United last season.
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