Webster, 55, brought forward his planned retirement and will leave with immediate effect.
In the interim non executive chairman John Dodds, who was appointed just three months ago, will assume the role of executive chairman while a replacement is found.
Dodds said: “I would like to thank Dean for his considerable contribution to Sweett Group over the last 34 years, the last nine of which as CEO.
“He has been instrumental in growing the business into the global operation it has become and we wish him every success for the future.”
Dodds, who is also non executive chairman at steelwork contractor Severfield, also took up the executive role for a short period last year while a new ceo was selected to replace ousted chief executive Tom Haughey.
Last week, Webster announced the appointment of Kenneth Wood as new executive operations director and Alan Manuel as deputy managing director for Europe.