The businessman and star of BBC One's The Apprentice said that after his two-year tenure his work was "complete".
The service offers 70 live Freeview channels and is a joint venture between the BBC, ITV, BT, Channel 4, Channel 5, TalkTalk and Arqiva.
YouView said Lord Sugar would go "in the next few weeks" and a new chairman would be announced in due course.
Formerly known as Project Canvas, YouView is a free web-connected TV service combining Freeview digital channels with on-demand content such as iPlayer. It is accessed via a set-top box.
Lord Sugar said: "When I joined YouView in 2011, my brief from the shareholders was to use my experience in the electronics industry to get the product to market and into the homes of consumers.
"Having done that successfully, my job is complete and I will now focus my time and attention into my other businesses."
YouView faces competition from rivals such as Sky, which has an on-demand service, and Virgin Media with its Tivo service.

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Lord Sugar is to step down from his role as non-executive chairman of internet TV service YouView.
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