The 29-year-old lock forward has signed from Glasgow Warriors, where he has made 87 appearances since 2010. Ryder, who has two caps for Scotland, began his career in the Leicester Tigers Academy before moving to Saracens in 2006.
He is available for immediate selection.
Director of rugby Jim Mallinder said that Ryder would be a useful addition to the squad for what will be a challenging last third of the season.
“We have a number of significant injuries in the second row, and Tom’s experience and quality will be useful to have around the club over the next few months,” he said. “We have difficult Aviva Premiership matches and a Champions Cup trip to Clermont Auvergne, and we’re confident that Tom will be able to contribute fully.”
Mallinder also thanked Ross McMillan for his contribution to the club in his 43 appearances over the past three years, the hooker having had his signing for Bristol announced on Monday by the Championship leaders.
“Ross was a good man to have around Franklin’s Gardens, who never gave less than 100 percent,” Mallinder added. “He moves on with our best wishes, and we hope that he will enjoy both his rugby at Bristol and being closer to his family.”