Ashwin, who is ranked third in the International Cricket Council's Test bowling rankings, will replace John Hastings as the Pears' overseas player for the final four games of the Specsavers Division Two campaign.
The Board of Control for Cricket in India has not always been keen to sign off its star players for domestic tournaments overseas, but with a high-profile Test series in England next summer it is seen as beneficial for Ashwin to gain extra experience here.
He had already been left out of India's one-day series in Sri Lanka, opening the door for Worcestershire.
"Ravi is a fantastic international cricketer who is currently in terrific form with his game," said director of cricket Steve Rhodes.
"He is ranked in the top three as a bowler in Test cricket, is a wonderful all-rounder who can bat six and scores hundreds in Test cricket.
"It is unlucky for John Hastings with his injury but Ravi is a wonderful cricketer for us to have at a crucial point in our season.
"It is a really important last few games in the Championship and to have someone of his class is a huge boost for the club.
"The good thing about Ravi is he has been successful on all types of pitches in all sorts of different countries. He is a class bowler who will bowl well here."
Ashwin brings with him a fine record of 286 wickets in 51 Tests, to go with 150 one-day international wickets and 52 in T20 internationals.
Ashwin played just two Tests in England on India's last tour three years ago, going wicketless at Old Trafford and taking three for 106 at the Oval.
India lost 3-1 on that trip, though Ashwin was the leading wicket-taker on home soil when the roles were reversed this winter, taking 28 in a 4-0 success.
Related links
Sky Bet's latest cricket odds
