The account was previously held by Elvis.
Proximity will take responsibility for all of the low-calorie sweetener's integrated marketing activity. Its first task will be to produce a programme promoting Splenda's partnership with the 2008 Olympic Games.
The agency has also been briefed to produce a campaign featuring the brand's frontman, the celebrity chef Anthony Worrall-Thompson.
The win reinforces BBDO's relationship with Splenda after the network secured the $35 million advertising account in July last year. The agency landed the business after a shoot-out against DDB, JWT, Mother and Alchemy.
Splenda - part of McNeil Consumer Nutritionals, a division of Johnson & Johnson - was the second-biggest brand in the artificial sweetener category last year, with a 17 per cent market share.