Deivid Washington only made his senior debut for Santos in April 2023
Washington, 18, has signed a seven-year contract at Stamford Bridge with the option of a further year.
He is Chelsea boss Mauricio Pochettino's ninth signing of the summer transfer window.
"I'm very happy to be joining a huge team like Chelsea," Washington told the club's website.
"I can't wait to make my debut here at such a big club, and provide lots of goals and assists."
Washington began his career with Gremio before he joined Santos in 2016.
His goalscoring in the youth ranks at Santos saw the youngster force his way into the first-team setup at the Brazilian club this year.
The versatile forward subsequently made a total of 16 senior appearances for Santos in all competitions and scored two goals before he made the move to Stamford Bridge.
He is the second player from Santos to join Chelsea this summer following the arrival of 18-year-old winger Angelo last month.
Pochettino said Moises Caicedo is "ready" to start his first match for Chelsea since his £100m move to the club from Brighton.
Chelsea face Luton on Friday night in the Premier League at Stamford Bridge.
Caicedo made his debut in Chelsea's 3-1 loss at West Ham on Sunday and gave away a penalty in a disappointing first outing.
Romeo Lavia, who joined the club for £58m from Southampton, will not be part of the squad for the visit of the newly-promoted side.
"Moises is ready to start. Lavia no. He is still, after they assessed him, he needs a few weeks to be ready to be involved with the team" Pochettino said.
"He is reconditioning now and working to try to catch the team and the team-mates. For Romeo we need to wait a few weeks until he is involved with the team."
Chelsea are winless after their first two Premier League games - a 1-1 draw with Liverpool in their season opener followed by the defeat to the Hammers.
The Blues created a number of chances against David Moyes' side but, despite their dominance in the first half, were not able to take them.
Chelsea's struggles in front of goal have come under scrutiny again after they only managed to find the net 38 times in the Premier League last season.
Pochettino's problem is compounded by the loss of £52m signing Christopher Nkunku to a knee injury which will keep him out until the end of the year.
Carney Chukwuemeka, who scored in the match against West Ham, has also been ruled out for a around six weeks with knee problem of his own.
Chelsea have been linked with a number of strikers and Pochettino said the club may still sign another attacking player as they continue to heavily invest in their squad.
"We are assessing the squad and all that happens. It is easy to say that we need an offensive player, but we have offensive players but they need to recover," added the Argentine.
"We need to wait with Armando [Broja]. Sometimes you are close and the last step is always difficult. He is training well and we have hope for him also.
"If now we bring in some player that is not fit or needs to adapt to the Premier League then maybe you are going to spend the same time that you need to wait for Broja or Nkunku or for Carney.
"Then you are going to create a mess in the squad. We are working really hard to analyse the situation and take the best decision.
"It is a little bit unlucky as maybe we need a little bit of help now. We are trying to find the right profile.
"We don't want to stop the evolution of some young guys that could be involved in the team."
--- Courtesy of BBC Sport