He spent last season on loan over with Dutch club Vitesse Arnhem.
A Chelsea statement read: “Izzy Brown has today signed for Rotherham United on loan until the end of the season.
“The attacker captained our Under-19s to UEFA Youth League glory in April 2015 and a month later made his first team debut against West Brom. He has also won the FA Youth Cup twice at Chelsea, in 2014 and 2015.
“Brown, an England Under-19 regular and a European Championship winner with his country at Under-17 level, spent last term on loan at Vitesse Arnhem.
“He will now continue his development with the Championship side managed by Alan Stubbs.”
The 19-year-old forward joined The Blues back in 2013 from West Brom just after making his senior debut for the Baggies.