The 31-year-old signed a two-year contract in March, but played only twice before today's bombshell announcement.
Eastmond has told Rhinos he no longer has the "passion" needed to play at Betfred Super League level.
His news comes less than a week after the Evening Post revealed another half-back, Rob Lui, will be leaving Rhinos at the end of this season.
Eastmond began his career with St Helens, then spent a decade in rugby union with Bath, Wasps, Leicester and England before signing for Rhinos.
He made his debut against Wigan Warriors on April 15 and also featured in the following week's defeat at Hull KR, but missed Rhinos' game at Huddersfield Giants with a hamstring problem and was not selected against Wakefield Trinity last Friday.
Eastmond said: “I love rugby and will always be involved in it.
"I’ve come to realise the passion to influence the game is no longer on the field.
"It is this love and respect for the game that leads me to this decision.
"I would like to say thank you so much to [director of rugby] Kevin Sinfield, Leeds Rhinos and all the fans and wish the team all the best for the rest of the season.”
Sinfield said Eastmond's decision was "incredibly brave" and "a testament to the high standards he sets for himself".
He added: "When we originally spoke to him, we had belief in his ability and talent and that remains the case, however after a year out of rugby due to the pandemic, he has found that he is unable to reach the standards he expects of himself in training and playing.
"He will leave the club with our best wishes for the future.
"The fact he had a contract for this year and next but decided to hang up his boots speaks volumes about his integrity.”