The highly-rated midfielder only joined The Black Cats in the summer in an acrimonious switch from Tranmere Rovers.
Thornton has yet to make his first team debut for the Premiership outfit but new manager Howard Wilkinson is keen for the Irishman to garner first team experience.
Seasiders manager Steve McMahon could not conceal his delight at clinching the youngster's signing and Thornton will initially be at Bloomfield Road for a month.
"Sean is a young, up and coming star," McMahon told the club's official website.
"I am delighted that he has joined us, he will benefit from playing first team football and will be a good addition to our squad.
"He played at this level last season including the 1-1 draw here at Bloomfield Road. He is an inventive, energetic midfielder who impressed me on a number of occasions last term."
Meanwhile, The Seasiders have finally completed the permanent signature of midfielder Keith Southern for an undisclosed fee from Premiership side Everton.
The 21-year-old has signed a two-and-a-half year deal with the Bloomfield Road outfit after impressing manager Steve McMahon during his three months on loan with the club.
Southern received an option for an extension with his new deal and The Toffees will receive a sell-on fee should Blackpool sell the youngster to another club.
On the signing of Southern, a delighted McMahon added: "He is a great acquisition for the club, given his age and given that we know all about him, we have had first hand experience of him.
"I think it is a great signing, we have stolen him in this day and age, he can only go one way, he is a good player. He is not spectacular but you do not need to be spectacular to be a good player, Keith has all the attributes of a good player."
In other news, Blackpool press officer Graham Emmerson has confirmed to planetfootball.com that on-loan defender Peter Clarke has returned to Everton following his three month spell at Bloomfield Road.
