The 21-year-old, who has helped Yorkshire reach NatWest T20 Blast Finals Day and the quarter-finals of the Royal London One-Day Cup, will remain part of the Vikings' limited overs squad but goes straight into Essex's Championship squad to play Derbyshire this weekend.
Rhodes made his Yorkshire debut in 2013, going on to captain England at the 2014 Under-19 World Cup.
He has played 15 first-class matches, 18 one-day games and 18 Twenty20s for Yorkshire and will now get a chance to play red-ball cricket as Essex chase promotion to Division One.
Essex head coach Chris Silverwood is pleased to welcome Rhodes to the club and confirmed he will play against Derbyshire.
"Will is an exciting young all-rounder who we hope will contribute with both bat and ball for the remainder of the season," he said.
"He is very highly thought of at Yorkshire but they are keen for him to get out and play some red-ball cricket, so we are pleased he decided to join the club. He will go straight into the squad for Saturday and we will pick a team according to the conditions."
Yorkshire Director of Cricket Martyn Moxon added: "We feel, as does Will, that this is a great opportunity to play first-team cricket and to improve as a player.
"It will be a great experience for him and hopefully will help to develop him further as an all-round cricketer."
© Cricket World 2016