The 22-year-old stopper arrives from Belgium club KRC Genk on a season-long loan with a view to a permanent deal.
Grzegorz started his career at hometown club Jagiellonia Bialystok and spent five years there before securing a move to Belgium to play for Genk in 2011.
He was unable to break into their first team so returned to Jagiellonia Bialystok in 2012 where he played 13 times towards the end of last season.
On the national stage 6'5" Sandomierski has three senior caps to his name and has also honours at under 21 level. This summer he was part of Poland's Euro 2012 squad although he didn't make an appearance at the tournament.
Speaking exclusively to Rovers Player
, the giant goalkeeper was delighted to make the move to England.
"I am very happy to be at this big club," said Grzegorz. "I think it will be a good place for me to improve and I hope to play a part in this big season.
"England is a special country, football is like a religion so I want to be here and I want to do well."
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