Goalsays Saints have offered a three-year contract offer which the 19-year-old has accepted.
The transfer will be finalised in the coming days.
Given Simeu's potential, the Blues have also inserted a hefty sell-on fee into the deal.
Simeu is part of a growing exodus amongst Chelsea academy players.
Tino Livramento, Lewis Bate and Myles Peart-Harris are also trying to leave the club this summer.
Leeds United are reportedly close to signing Bate, while Brighton have an interest in Livramento.
Brentford are admirers of attacking midfielder Peart-Harris.