The Northampton number eight takes over the reins from Test skipper Chris Robshaw, who leads Harlequins against Leicester in the Aviva Premiership final the previous day.
Dowson has been a senior figure in Stuart Lancaster's England squad, even after losing his place in the starting side to Ben Morgan during the RBS 6 Nations.
With Morgan recovering from a hamstring injury, Dowson will start against a star-studded Barbarians side and he has been chosen as captain ahead of his club skipper Dylan Hartley.
"Phil is one of the key members of out leadership group. He will captain the team on Sunday," said England forwards coach Graham Rowntree.
Hartley is set to play his first game since the Six Nations after serving an eight week ban for biting Ireland flanker Stephen Ferris.
Rowntree believes Hartley is a future England captain but this weekend wanted the Northampton hooker to concentrate on his scrummaging.
"The fact he has been out for eight weeks, I want Dylan to focus on playing, to focus on his game and not be saddled with captaincy issues," Rowntree said.
Dowson's elevation is a reward for his leadership during the Six Nations as a new-look England team defied the odds to finish second in the championship.
And that ability to galvanise a new team is particularly important this week, with 18 members of the touring squad absent on Premiership final duty.
"Phil coped with the disappointment (of losing his place in the team) very well. That is the mark of Phil Dowson," Rowntree said.
"He took it on the chin and we all saw the rise of Ben Morgan towards the end of the Six Nations so it wasn't easy for him.
"But he took it on the chin and never stopped giving his experience to the younger guys and giving to the group. He is a great leader.
"When he makes a point it is worthwhile. We have a strong leadership group at the moment. There are a few guys we could give the captaincy to but we feel Dows is right for this game.
"When you are pulling guys together for the first time, that leadership is important. He will make sure we are right on it.
"This is a proper game, as intense and as big a game as a Six Nations game for us."
England are using the traditional Barbarians fixture as a launch-pad for their challenging five-match, three-Test summer tour to South Africa.
Whatever liquid preparations the Barbarians enjoy this week, Rowntree is determined that England treat the game as if it were a Test match.
Unlike the Welsh Rugby Union, England do not award caps for matches against the Barbarians although Rowntree would not be against the idea.
"I think it's a serious enough game to be capped. Maybe that is a consideration going forward. We are treating it as if it's a capped game," Rowntree said.
England want to build on the momentum they gained in the Six Nations.
One player to catch Rowntree's eye in training has been Bath flanker Carl Fearns, one of the uncapped members of England's 42-man tour party.
"There is a real energy in the squad," Rowntree said.
"It is an environment where people are clamouring to be part of it. The guys that have come in have led that energy in training, particularly Fearnsy.
"He finished the season very well for Bath. I give him a special mention. I have been very impressed with him.
"This group have the same desire and energy to play for their country (as we had in the Six Nations).
"I am already feeling there are a lot of guys who feel this is where they want to be."