The skilful left-footed attacker has been on the radar of the Spanish team, which finished mid-table last season, for some time with the official unveiling Thursday 5pm local time (Friday 1am AEST) at the club's Los Pajaritos home.
Naumoff, 20, was out-of-contract at the end of the 2015-2016 A-League season - in which he made a total of 18 appearances across all competitions for the Sky Blues.
Naumoff played in six of Sydney's eight AFC Champions League matches this year and scored two goals in the continental competition as the team qualified for the Round of 16 for the first time in their history.
Joining the Sky Blues in 2012, Sydney-born Naumoff was named the club's National Youth League player of the year in 2014 and after making his first-team debut in Round 1 of the 2013-2014 campaign played 44 A-League matches across three seasons - scoring twice.
Naumoff has represented Australia at U-20 and U-23 level and will join the likes of Socceroos No. 1 Mat Ryan among the Aussies abroad playing their trade in Spain.
Numancia, which finished 10th in Spain's second tier last season, is coach by 38 year-old Jagoba Arrasate.
