The 22-year-old has penned a four-year deal with Derby and will link up with fellow Polish international Krystian Bielik.
Although no fee was disclosed, reports suggest it to be in the region of €4.3m (£4m).
Derby boss Phillip Cocu could call on Jozwiak as early as Saturday when the Rams visit Luton Town.
Jozwiak made 122 appearances for Lech Poznan, scoring 17 and assisting 15 goals.
He has been capped three times by Poland and started their recent internationals against the Netherlands and Bosnia.