The former Everton and Millwall midfielder signed for the American outfit in 2012 - picking up a host of individual honours as well as helping Red Bulls to the Supporters' Shield in 2013.
However, after Cahill helped Australia to Asian Cup success on Saturday, the 35-year-old explained he was looking forward to the next phase of his career.
"I've enjoyed every moment of my time at the Red Bulls and leave with great memories," he told the club's official website.
"I joined with the ambition of helping to grow and positively affect soccer in the US and I feel immensely proud of what we achieved as a team during my time in the MLS.
"Winning the Supporters' Shield was, without doubt, one of the best moments I have experienced in my career and I am now looking forward to my next chapter, of which I will announce in due course."
