The 21-year-old, who has still to make his competitive debut for the Championship outfit, spent last season on loan at Aldershot and Cheltenham.
Aberdeen manager Derek McInnes explained that he has injury concerns in defence, with centre-half Andrew Considine still short of match fitness.
"Michael's arrival concludes things as far as players coming in is concerned," McInnes told his club website.
McInnes, who lost Clark Robertson and Russell Anderson to minor injuries during the midweek friendly defeat at Peterhead, described London-born Hector as "a big strong lad who comes with a good pedigree".
The Dons become the 11th club the central defender, who is eligible to play for Jamaica, has joined on loan during his time with Reading.