The independent digital specialist triumphed following a competitive pitch with a string of rivals, thought to have included incumbent Mediaedge:cia and MediaCom, as well as Unique.
MediaCom recently won the £8m offline account for Bulldog, which is planning an aggressive marketing blitz to plug its high-speed broadband service.
Neither Profero or Bulldog would comment on the win, but it caps a great week for the digital agency which, only a few days ago, walked off with the digital media planning and buying account for Channel 4.
Profero will handle the online media for the broadcaster's string of channels as well as its growing new media arm.
Alex Jeffries, media and planning director at Profero, said about the C4 account win: "To win Channel 4 after such a competitive pitch, which I think at one stage had something like 11 agencies involved, is great news for us. It's also a perfect time for us to win it."
C4 has spent up to £2m in online marketing but looks set to increase its billings, as it puts more resources into digital media.
Chief executive of C4, Andy Duncan, has said that new media is at the heart of the channel's strategy and it is introducing broadband TV content, as well as new services in areas such as music downloading.