The Razorbacks (4-3) fell behind by nine, 87-78, on two Frank Williams free throws with 2:09 to play, but Dean scored five of his career and game-high 29 points over the next 33 seconds to get UA within 87-83.
Williams, last year's Big Ten Player of the Year, made three more free throws but freshman J.J. Sullinger, who was playing in front of friends and family from his hometown of Columbus, Ohio, converted a three-point play to cut the deficit to 90-87 with 58 seconds remaining.
Arkansas forced Illinois into one of its season-high 22 turnovers and Jannero Pargo, a native of Chicago who had 30 friends and family members at the game, hit his seventh three-pointer of the game to put the Hogs in front, 91-90.
Damir Krupalija scored to give Illinois the lead but Dean drove inside and made a layup with six seconds remaining, but was called for charging with the bucket waved off.
Williams, who made just three field goals but was 18 of 24 at the line for a team-high 25 points, made two free throws in the final five seconds for the final margin.
Dean set career highs for points scored, shots taken (19) and shots made (11) while Pargo matched his career scoring high of 28 points and made seven three-pointers for the second straight game. Pargo, who added six assists and four steals, was nine of 17 overall, matching career highs in each category, and seven of 13 from behind the arc.
Sullinger added 13 points, four rebounds, four assists and two steals. Redshirt freshman Michael Jones played a season-high 22 minutes and had season bests of nine points and five rebounds.
The loss denied head coach Nolan Richardson his 500th career victory. Richardson goes for win No. 500 on Monday, Dec. 10, when the Razorbacks play host to North Carolina-Greensboro at 7 p.m. in a game televised by SEC-TV and carried in Arkansas by Fox Sports Southwest.