Man-of-the-match Bell's innings at Lord's on Sunday was the cornerstone of England's 224 for four as they won with 20 balls to spare after good work by their bowlers restricted South Africa to 220 for eight.
Together with Warwickshire colleague Jonathan Trott (48), Bell shared a second-wicket stand of 141 after England lost captain Alastair Cook in the first over of their innings when he was lbw to a superb late in-swinger from fast bowler Dale Steyn.
But the Proteas – without rested paceman Morné Morkel – couldn't make further early inroads despite Steyn hitting Trott, a former South Africa youth international, on the finger.
As the floodlights took full effect on a gloomy day where the overcast conditions had prompted Cook to field first, Bell cover-drove left-arm seamer Wayne Parnell for four and advanced down the pitch to flat bat him over cover-point for another boundary.
Bell later drove left-arm spinner Robin Peterson high over long-off for six.
He was reprieved though on 65 when dropped by Peterson at mid-on off Lonwabo Tsotsobe. Next ball Bell pulled the left-arm seamer for four.
"I've been working at Warwickshire at hitting over the top, that's what you need to do opening the batting in one-day internationals," explained Bell.
Trott's patient innings ended when he was lbw on the sweep to slow left-armer Dean Elgar after South Africa reviewed Australian umpire Simon Taufel's original not out decision.
After Ravi Bopara fell cheaply following a brief rain break, Bell tried to slap a short ball from Steyn on the offside and was caught behind one-handed by leaping South Africa's captain and wicket-keeper AB de Villiers to end a 137-ball innings featuring a six and eight fours.
England still needed a further 35 runs for victory but the innings ended in a blizzard of boundaries, with left-hander Eoin Morgan a run-a-ball 36 not out and Craig Kieswetter, who ended the match with a superb off-driven six off Steyn, unbeaten on 21.
Earlier, James Tredwell atoned for dropping two catches with three wickets as England restricted South Africa to 220 for eight.
The off-spinner took three for 35 in eight overs, with all his wickets stumpings completed by Kieswetter – the first England wicket-keeper to have three stumpings in a one-day international.
"We've got a lot to look at now; our middle order failed again," said a disappointed de Villiers.
"We keep getting in and fail to go on to big runs. We just didn't have enough runs on the board today."
Cook added: "We made the most of the early conditions. Finny [Steven Finn] and Jimmy [Anderson] up front asked plenty of questions."
But the new-ball duo were denied an early wicket.
Tredwell, who took two for 49 in England's four-wicket series-levelling win across London at The Oval on Friday, got his chance this campaign when England rested No. 1 spinner Graeme Swann because of a longstanding elbow injury.
And it seemed England might miss Swann as much for his slip catching as his bowling on Sunday when Tredwell dropped openers Hashim Amla, on four, and Graeme Smith, on 26, both off fast bowler Finn.
Dropped catches had helped Amla become the first South African to a Test triple century, at The Oval last month, and make 150 in an 80-run win in the second one-dayer in Southampton.
As it was, missing Amla on Sunday cost England 41 runs when he was bowled by medium-pacer Bopara for an innings top score of 45.
Amla's exit was the start of a South Africa collapse from 100 for one to 115 for four with JP Duminy stumped off a well-flighted Tredwell delivery.
De Villiers (29) and Parnell (5) were also both stumped by Kieswetter as Tredwell took two wickets for no runs in five balls.
The series concludes at Nottingham's Trent Bridge ground on Wednesday. – AFP