The player had been presented alongside other Leganes player to fans on Tuesday evening but his deal from the Blues was officially announced on Wednesday morning, with an official unveiling set to follow on Friday.
Prior to the start of the 2017-18 campaign, Omeruo - who once revealed his frustration at lack of opportunities at Chelsea - renewed his contract with the former English top-flight champions till 2020 and was immediately shipped out for another spell at Kasimpasa.
The Nigeria defender made 28 Turkish Super Lig appearances, scoring a goal in the process.
His form also saw him make Nigeria's squad to the 2018 Fifa World Cup this summer where he played twice, impressing in the Super Eagles' win over Iceland.
The temporary move to the Spanish top-flight represents his seventh since signing for the London outfit from Standard Liege in January 2012.
And he will hope to feature prominently in Mauricio Pellegrino's setup - and hopefully, impress new Chelsea coach Maurizio Sarri enough to get his elusive competitive debut at his parent club.
Leganes start their 2018-19 LaLiga campaign with a tie against Athletic Bilbao next Monday.