Dent, whose previous top score in List A cricket was 36, finished 151 not out after hitting 17 boundaries and three sixes in a 113-ball knock. His innings helped the visitors to 288 for five at the SWALEC Stadium before the bowlers kept Glamorgan to 281 for six.
Mark Wallace was unlucky to finish on the losing side, carrying his bat for 118no, and might have won it for Glamorgan had James Fuller (three for 42) not removed his sidekick Graham Wagg for 54.
Table-toppers Somerset put their poor first-class form behind them by making it three wins from three in the shorter form with a six-wicket defeat of Middlesex. Middlesex made 252 for nine batting first, Joe Denly top-scoring with 60 and Dawid Malan making 49 at the head of the innings.
Four wickets from Steve Kirby and three from Max Waller kept the total in check, though, and Somerset crossed the line four down with 25 balls remaining.
Peter Trego (81) and Arul Suppiah (60) did the bulk of the scoring, with England one-day wicketkeeper Jos Buttler adding the finishing touches with 23 in 12 balls - including a six to win the match.
Elsewhere in Group C, Adam Lyth's unbeaten half-century carried Yorkshire to a first win of the season in the competition, at the expense of the struggling Unicorns. The Unicorns made 189 for nine - number nine Aneurin Norman top-scoring with 38. Gary Ballance's 44 made inroads into the total and Lyth, with 58no, reeled in the target with seven balls left.
Group B leaders Hampshire recorded a third win from four matches as Michael Carberry's 96 saw them to a five-wicket success over Durham at the Rose Bowl. Durham managed 241 for eight in a reduced 36-over innings, all-rounder Ben Stokes hitting six fours and four sixes in a quickfire 87.
Chris Wood and David Griffiths each took three wickets but Durham's bowlers could not enjoy the same penetration - largely thanks to Carberry's 86-ball stay, which ended with one run needed. Jimmy Adams helped out with 60 and Liam Dawson's 37no saw them safe with five balls left.
Lancashire beat Surrey by seven runs at Old Trafford in a low-scoring game. The Red Rose scored 176 for nine, with Karl Brown playing a near lone hand with 80 in 84 balls, and that proved enough. Surrey subsided for 169, Stephen Parry with career-best figures of five for 17 and Wayne White with four for 35.