The Brazilian joined Chelsea from Sao Paulo in 2011, but has failed to break into the first team at Stamford Bridge with any frequency.
His one Premier League appearance came as a substitute in an 8-0 win over Aston Villa in December 2012, and the 20-year-old has had spells out on loan in a bid to experience regular football.
He was loaned to Malaga for the second half of the 2012-13 campaign, where he made 11 La Liga appearances, four of which were starts.
Last season, Piazon was loaned to Vitesse in the Eredivisie, where he scored 11 goals in 29 league showings.
"Lucas Piazon has today joined German side Eintracht Frankfurt on a season-long loan," a statement on Chelsea's website read.
"The 20-year-old will continue his impressive progress by joining the Bundesliga side."