The 18-year-old Brazilian midfielder will link up with the Belgian Pro League club for the remainder of the season in what will be his second loan stint since arriving at Anfield in September 2015.
Allan, who joined the Reds from Internacional, starred for SJK by scoring two goals and providing five assists in helping the club win their first-ever Finnish title late last year.
The promising youngster will go straight into Chris O'Loughlin's squad for this weekend's league meeting with Westerlo. Sint-Truiden currently sit 10th on the table.
#LFC can confirm that Allan Rodrigues de Souza has agreed a loan move to Sint-Truidense until the end of the season pic.twitter.com/rWBfDOwWYM
— Liverpool FC (@LFC) January 18, 2016