The Padgett's Motorcycles rider, who became the fastest 250 racer ever at the Lightweight Classic TT in 2016, has never ridden in the class at Dundrod.
"I started my racing career on a Suzuki RGV250 and I still just love racing the 250," said Anstey.
"I can't wait to get the RCV out at Dundrod - the 250 is a thoroughbred racing bike."
Team boss Clive Padgett has also confirmed that Anstey will ride iconic Moto GP bike, the Honda RC213V-S, at the Dundrod circuit for the second year running.
"We're proud to be the only one of the three international road races to run an ultra lightweight/lightweight class," said Noel Johnston, Ulster GP clerk of the course.
"It's great to see a rider like Bruce entering this class for no other reason than he really enjoys it.
"I think it'll give the race a bit of a shake up, as Bruce's calibre and track record combined with Clive Padgett's experience and machinery will surely be very hard to beat.
"It's been a somewhat subdued season for Anstey so far, but if there's one thing I've learned, it's to never ever discount him for podium contention at the Ulster."
New Zealand's Bruce Anstey has announced he will ride a two stroke in the Ulster Grand Prix next month.