Smith's experience was still vital to the team as he was South Africa's leading run scorer in the shortened format of the game, South Africa's coach Corrie van Zyl said in Bloemfontein on Wednesday.
Smith stood down as captain of the T20 side last month, but will stay on as captain of the One Day International (ODI) team until after the World Cup in India, Sri Lanka and Bangladesh in February next year.
Johan Botha is the new T20 captain.
"We picked Graeme because he's got one of the best records in T20 cricket," said Van Zyl.
"It's also important for us to get the guys playing as much cricket as possible at this stage of the season and, of course, it's also good to have the one-day captain playing in this format as well."
Smith is the third highest run scorer (803 runs from 27 matches) in Twenty20 cricket, after New Zealand's Brendon McCullum (1100 from 40) and England's Kevin Pietersen (911 from 28).
Van Zyl said the series against Zimbabwe was the start of an important season for the Proteas and they would not be taking their neighbours lightly.
"It's a very important season for us and it's going to be a very busy season. We see the series against Zimbabwe as a stand-alone series, but it's also part of a whole campaign and a whole strategy towards the World Cup.
"Every one-day match we can play now and every Twenty20 is geared towards getting our skills honed for the World Cup but, having said that, it's also very important that we see this series, particularly the T20 series starting on Friday night, as a series against Zimbabwe."
The visitors, coming off a recent series win against Ireland, have never before played South Africa in a T20 game.
Their first meeting, at 18:00 on Friday, takes place at the same ground in Bloemfontein (OUTsurance Oval) where they met for their first Test match in October 1999.
Van Zyl said South Africa did not see Zimbabwe as underdogs and had prepared accordingly.
"We can never take a team lightly. We've prepared for this series like we would do for any other team in terms of our statistical and opposition analysis so we're definitely not taking them lightly.
"Having said that, we also need to make sure we concentrate on our own skills and what we need to do right come Friday night."
Van Zyl said injuries to Jacques Kallis and Dale Steyn were a concern, but they would have been rested for the T20 series anyway.
Albie Morkel's injury was a bigger disappointment for the Proteas, but Ryan McLaren is standing in for him as the all-rounder on Friday night.
Meanwhile, Van Zyl confirmed an A-list of former international players would be working with the Proteas for a couple of days in Bloemfontein from Monday.
Shaun Pollock, Lance Klusener, Jonty Rhodes, Shafiek Abrahams and Alan Dawson will join former England coach Duncan Fletcher as specialist consultants in their areas of expertise.