Birmingham agree Derbyshire deal
The 24-year-old made just 19 appearances for Olympiakos last season, scoring six goals in a spell that was hindered by injury and loss of form.
"Olympiakos announces that an agreement has been reached with Birmingham for the loan of Matt Derbyshire for one year," a brief statement posted on the club website said.
Derbyshire was left out of the Olympiakos squad by coach Ernesto Valverde for the 5-1 friendly win against Roma on Aug 13.
Valverde, who recently returned to the club after a one-year absence, signed Derbyshire on loan initially in 2009.
The former England under-21 international's highlight during his time in Greece was helping his club do the double by scoring twice in the Greek Cup final win over AEK Athens.