The 21-year-old front man arrives from French club Toulouse for a fee of £24m plus add-ons as the Blues prepare for life back in the Premier League.
“The signing of Emersonn is an exciting moment for the club," said Ashton.
“As we prepare for our return to the Premier League we know recruitment will be crucial, with the club investing significantly in this area in recent years in order to bring the best possible talent from across the globe to the club.
“In making Emersonn our first signing of the summer we know we have acquired a young, hungry player with a high ceiling, with more signings to follow as we continue to invest to give ourselves the best possible chance of achieving our goal in the Premier League.”