He agreed a two-year deal with the Province back in February but his impressive form this season has led to a further extension. The versatile 27-year-old has started 8 of Ulster's 9 games to date this season, operating at openside flanker and number 8 for Les Kiss' side.
Reidy is Irish-qualified through his grandfather and he made his international debut during Ireland's summer tour of South Africa back in June.
Commenting on his decision to put pen to paper, Reidy said:
"I'm really happy to be sticking around. I've enjoyed the past two and a half years a lot and my partner and I really love the place.
"I've been pleased with the amount of minutes I've got under my belt so far this season but I'm going to keep my head down and try to keep developing my game under Les and the rest of the coaches.
"The setup here is really good and I'm looking forward to being part of an exciting journey over the next few years."
Kiss welcomed the news that Reidy was extending his stay in Belfast:
"It's brilliant that Sean has committed to us until 2020; it's another good bit of business by our Operations Director, Bryn Cunningham.
"Sean has worked hard since his arrival at Ulster and his game has developed very well as a result of that. During the summer, I was delighted that his hard work was rewarded with a first international cap.
"His defence and breakdown work has been very impressive in the early stages of the season and that has had a big impact for us. I am really enthusiastic about what Sean can deliver for us over the next few years."