The Nigerian international, 24, joined the Blues in 2012 but is yet to play a senior game.
And Stamford Bridge chiefs have announced his latest short-term exit as he seeks to impress with the Spanish minnows.
Leganes have finished 17th two seasons in a row to keep their top-flight status despite a modest budget.
And they will look to kick on after racking up an impressive 43 points last time out – just eight off a top-half finish.
Omeruo signed an extension until 2020 before heading to Turkey with Kasimpasa last season, where he played 28 league games.
His form saw him called up into Nigeria's World Cup squad and he featured against Nigeria and Iceland – putting in a fine display against the Scandinavians.
Former Southampton chief Mauricio Pellegrino will be his boss in Spain, with Leganes beginning their campaign against Athletic Bilbao on Monday.
Since joining Chelsea from Standard Liege, Omeruo has been loaned out to ADO Den Haag, Middlesbrough, Kasimpasa (twice) and Alanyspor for a total of 154 senior appearances.
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