In addition to serving as the party's leader since 2011, Mr. Ryan is currently the Minister for the Environment, a position he will hold after resigning.
Mr. Ryan played a pivotal role in the party's comeback after losing the 2011 General Election.
But he has decided to resign following a dismal local and European election in which the Green Party lost both of its MEPs.
Mr. Ryan, a TD representing Dublin Bay South since 2016, also declared he will not be contesting in the upcoming general election, despite rumours that it may happen this year due to his resignation.
Taoiseach Simon Harris paid tribute to Mr. Ryan after the announcement on Tuesday, calling him a “genuine, passionate, and inherently decent person”.
"Minister Eamon Ryan notified me yesterday of his decision to step down as the Green Party's leader.
"Eamon is a highly esteemed politician, and I would like to honour him and his family for all that they have contributed to Irish politics by way of his Greens leadership. I accept and comprehend his choice.
"Eamon is a sincere, fervent, and morally upright individual who applies these traits to politics. "The Green Party's bylaws and members now have a say in who leads the organisation.
"The government is still in operation, and the three coalition parties' shared government agenda will be being carried out. "We need to do a lot of work."
Additionally, Tánaiste Micheál Martin expressed his admiration, saying, "Eamon is a man of integrity & commitment." Our relationship is positive on both a personal and business level.
"He has always been consistent in his personal beliefs in terms of climate & biodiversity."
Roderic O'Gorman, TD for the Green Party and Minister for Integration, commended Eamon Ryan for his accomplishments as the party leader.
"I'm so grateful to him for all that he's done, for his leadership of the party, for his commitment to advancing climate action and nature restoration."